The Making of Species :: CHAPTER VI THE COLOURATION OF ORGANISMS

CHAPTER VI THE COLOURATION OF ORGANISMS

The theory of protective colouration has been carried to absurd lengths—?It will not bear close scrutiny—?Cryptic colouring—?Sematic colours—?Pseudo-sematic colours—?Batesian and MÜllerian mimicry—?Conditions necessary for mimicry—?Examples—?Recognition markings—?The theory of obliterative colouration—?Criticism of the theory—?Objections to the theory of cryptic colouring—?Whiteness of the Arctic fauna is exaggerated—?Illustrative tables—?Pelagic organisms—?Objectors to the Neo-Darwinian theories of colouration are to be found among field naturalists—?G. A. B. Dewar, Gadow, Robinson, F. C. Selous quoted—?Colours of birds’ eggs—?Warning colouration—?Objections to the theory—?Eisig’s theory—?So-called intimidating attitudes of animals—?Mimicry—?The case for the theory—?The case against the theory—?“False mimicry”—?Theory of recognition colours—?The theory refuted—?Colours of flowers and fruits—?Neo-Darwinian explanations—?Objections—?Kay Robinson’s theory—?Conclusion that Neo-Darwinian theories are untenable—?Some suggestions regarding the colouration of animals—?Through the diversity of colouring of organisms something like order runs—?The connection between biological molecules and colour—?Tylor on colour patterns in animals—?Bonhote’s theory of poecilomeres—?Summary of conclusions arrived at.

Since the publication of The Origin of Species, naturalists have paid much attention to the colouration of animals and plants, with the result that a large majority of scientific men to-day hold the belief that all, or nearly all, the colours displayed by animals are of direct utility to them, and are therefore the direct result of natural selection; a few would add, “and of sexual selection.”

“Among the numerous applications of the Darwinian theory,” writes Wallace, “in the interpretation of the complex phenomena, none have been more successful than those which deal with the colours of animals and plants.”

Robinson on Protective Colouring

We readily admit that the Darwinian theory has thrown a great deal of light on the phenomenon of animal colouration; it has reduced to something like order what was before Darwin’s time chaos. While admitting this we feel constrained to say that many naturalists, especially Dr Wallace and Professor Poulton, have pushed the various theories of animal colouration to absurd lengths. As Dr H. Robinson truly says (Knowledge, January 1909), “It seems to have been taken for granted, and some even of Dr Wallace’s writings may be interpreted in this sense, that protective colouring is necessary to the continued existence of every species, and that, sexual colouration apart, it is incumbent on naturalists to offer ingenious speculations in this sense to account for the appearance even of the most bizarre and conspicuous beasts. Thence it has been but a short step to the announcement of those speculations as further evidence in favour of natural selection, and of various assumptions made in the speculative process as indisputable facts.”

The result of this is that men have ceased to regard the Neo-Darwinian[6] theories of protective colouration, mimicry, and recognition markings as mere hypotheses which seem to throw light on certain phenomena in the organic world. These theories have assumed the rank of laws of nature. To dispute them would seem to be as futile as to assert that the earth is flat. To take exception to them would appear to be as ridiculous as to object to Mont Blanc. To dare to criticise them is heresy of the worst type.

Be this as it may, scientific dogma or no scientific dogma, scientific opinion or no scientific opinion, we have dared to weigh these theories in the balance of observation and reason, and have found them wanting. We have examined these mighty images of gold, and silver, and brass, and iron, and found that there is much clay in the feet.

We shall devote this chapter to lifting the hem of the garment of sanctity that envelopes each of these images, and so expose to view the clay that lies concealed.

We propose, first, to set forth in outline what we trust will be considered a fair statement of the various theories of animal colouration which are generally accepted to-day, then to show up the various weak points in these, and lastly, to endeavour to ascertain whether there are not some alternative explanations in certain cases to which the generally-accepted theory does not apply.

Cryptic Colouring

Neo-Darwinians divide the various forms of colouration into three great classes:—(1) Cryptic colouring, or protective and aggressive resemblances; (2) sematic colours, or warning and recognition colours; and (3) pseudo-sematic colours, or mimicry. A tabular statement of this scheme of colouring will be found on pp. 293-7 Professor Poulton’s Essays on Evolution.

As regards class (1), Neo-Darwinians point out that the great majority of animals are so coloured as to make them very difficult to see in their natural environment, hence the whiteness of the creatures which inhabit the snow-bound Arctic regions, the sandy colour of desert animals, the spotted coats of creatures which live among trees, the striped markings of animals which spend their lives amid long grass, and the transparent blueness of pelagic animals. The theory is that all kinds of animals, whether those that hunt or those that are hunted, derive much advantage from being coloured like their environment. The hunted creatures are thereby the better able to elude the vigilance of their foes, while those that hunt are in a position to take their quarry by surprise; so that natural selection has caused them all to assimilate to the hues of their surroundings. Neo-Darwinians point to the fact that some Arctic animals are brown in the summer to match the ground from which the snow has melted, and turn white in winter to assimilate with their snowy background. Naturalists further cite, as evidence in favour of this theory, the case of those creatures which imitate inanimate objects, such as leaves and twigs, and thereby escape the observation of their foes.

Thus, the great majority of animals are supposed to be cryptically coloured, that is to say, coloured so as to be, if not quite invisible, at least very inconspicuous in their natural habitat.

Warning Colouration

It is, however, generally admitted that many creatures are not cryptically coloured. Some, indeed, seem to be coloured in such a way as to render them as conspicuous as possible. The Neo-Darwinians declare that there is a reason for this. “If,” writes Professor Milnes Marshall (page 133 of his Lectures on the Darwinian Theory), “an animal, belonging to a group liable to be eaten by others, is possessed of a nauseous taste, or if an animal, such as a wasp, is specially armed and venomous, it is to its advantage that it should be recognised quickly, and so avoided by animals that might be disposed to take it as food.

“Hence arises warning colouration, the explanation of which is due to Wallace. Darwin, who was unable to explain the reason for the gaudy colouration of some caterpillars, stated his difficulty to Wallace, and asked for suggestions. Wallace thought the matter over, considered all known cases, and then ventured to predict that birds and other enemies would be found to refuse such caterpillars if offered to them. This explanation, first applied to caterpillars, soon extended to adult forms, not only of insects, but of other groups as well. . . . Insects afford many admirable examples of warning colours, and many well-known instances occur among butterflies. The best examples of these are found in three great families of butterflies—the HeliconidÆ, found in South America, the DanaidÆ, found in Asia and tropical regions generally, and the AcrÆidÆ of Africa. These have large but rather weak wings, and fly slowly. They are always very abundant, all have conspicuous colours or markings, and often a peculiar form of flight, characters by which they can be recognised at a glance. The colours are nearly always the same on both upper and under surfaces of the wings; they never try to conceal themselves, but rest on the upper surfaces of leaves and flowers. Moreover, they all have juices which exhale a powerful scent; so that, if they are killed by pinching the body, a liquid exudes which stains the fingers yellow, and leaves an odour which can only be removed by repeated washing. This odour is not very offensive to man, but has been shown by experiment to be so to birds and other insect-eating animals.

“Warning colours are advertisements, often highly coloured advertisements, of unsuitability as food. Insects are of two kinds—those which are extremely difficult to find, and those which are rendered prominent through startling colours and conspicuous attitudes. Warning colours may usually be distinguished by being conspicuously exposed when the animal is at rest. Crude patterns and startling contrasts in colour are characteristically warning, and these colours and patterns often resemble each other; black combined with white, yellow, or red, are the commonest combinations, and the patterns usually consist of rings, stripes, or spots.”

We trust that we shall be forgiven for this lengthy quotation. Our object in reproducing so large an extract is to allow the Neo-Darwinians to speak for themselves. Were we to state their theory in our own words, we might perhaps be charged with stating it inaccurately. We should add that, even as natural selection is supposed to have been the cause of conspicuous colouring in some organisms, so has it caused others to assume intimidating attitudes or emit warning sounds, such as a hiss, when attacked.

Batesian Mimicry

We now come to the third great class of animal colours—mimetic colours. Mimicry is of two kinds, known respectively as Batesian and MÜllerian mimicry, after their respective discoverers.

It has been found that some apparently warningly coloured butterflies and other creatures are palatable to insectivorous animals. The explanation given of this is that these showy but edible butterflies “mimic,” that is to say, have the appearance of, show a general resemblance to, species which are unpalatable. This is known as Batesian mimicry. “Protective mimicry,” writes Professor Poulton (Essays on Evolution, p. 361), “is here defined as an advantageous superficial resemblance of a palatable defenceless form to another that is specially defended so as to be disliked or feared by the majority of enemies of the groups to which both mimic and model belong—a resemblance which appeals to the senses of animal enemies . . . but does not extend to deep-seated characters, except when the superficial likeness is affected thereby.”

As Wallace has pointed out, five conditions must be satisfied before such protective mimicry can occur:—

“1. That the imitative species occur in the same area and occupy the same station as the imitated. 2. That the imitators are always the more defenceless. 3. That the imitators are always less numerous in individuals. 4. That the imitators differ from the bulk of their allies. 5. That the imitation, however minute, is external and visible only, never extending to internal characters or to such as does not affect the external characters.” (Darwinism, Chap. ix.)

Thus the mimic is supposed to deceive his enemies by deluding them into the belief that he is the inedible species which they once tried to eat and vowed never again to touch, so nasty was it. The mimic, then, may be compared to the ass in the lion’s skin. Needless to say, this mimicry is quite unconscious. It is supposed to have been developed by natural selection. Every popular book on Evolution cites many examples of such mimicry. We may therefore content ourselves with mentioning but a few.

Examples of Mimicry

Our common wasps are copied by a beetle (Clytus arietis), active in movement and banded black and yellow, and by several yellow-barred hover-flies (SyrphidÆ); and the bumble-bee by a clear-winged moth (Sesia fuciformis). There is, indeed, a whole group of these clear-winged moths, resembling bees, wasps, and other stinging hymenoptera. The common Indian Danaid butterfly, Danais chrysippus, is marvellously reproduced by the female of Hypolimnas misippus, a form allied to our Purple Emperor. The male of this is black, with white blue-bordered patches, the female chestnut, edged with black and with white spots at the tips of the wings, as in the Danais. Finn has shown experimentally that this species is liked by birds.

Another common Indian Danaid (D. limniace), black, spotted with pale green, is imitated, though not very closely, by the female of one of the “white” group, Nepheronia hippia. Finn found that this insect was eaten freely by birds, and that the common jungle-babbler (Crateropus canorus) was deceived by the mimicry of the female. The very nauseous Indian swallow-tail (Papilio aristolochiÆ) is closely imitated by another swallow-tail (P. polites), both having black wings marked with red and white; P. aristolochiÆ, however, has a red abdomen. This difference was not noticed by two species of Drongo-shrikes (Dicrurus ater and Dissemurus paradiseus), to which the butterflies were offered; but the Pekin robin (Liothrix luteus)—a very intelligent little bird—did not fail to pick out and eat the mimic, though it was deceived by the marvellously perfect imitation of Danais chrysippus, by the female of the Hypolimnas.

Such resemblances can therefore be effective.

The cases of mimicry usually quoted include very few among mammals, probably, as Beddard suggests, because the species of that class are relatively few.

The insectivorous genus Tupaia is supposed to mimic the squirrels, which it much resembles as regards form in all respects save the long muzzle; the idea being that squirrels are so active that carnivorous animals find it hopeless to pursue them.

On the other hand, there is a squirrel (Rhinosciurus tupaioides) which is supposed to mimic the tupaias! It has a similar long muzzle, and the light shoulder-stripe which is a common marking in tupaias. But why the squirrel, one of the group imitated, should in turn become an imitator is not explained.

The true interpretation of the resemblance is probably that both squirrels and tupaias are adapted to a life in trees. Like profession begets like appearance: the ground-living shrews much resemble mice, and the moles find representatives in mole-like rodents.

Another case, however, wherein true mimicry may have come into play is that of the South American deer (Cervus paludosus) which singularly resembles in colouration the long-legged wolf or Aguara-guazu (Canis jubatus). Both these species are chestnut in colour, with the front of the legs black, and the ears lined with white hair; both inhabit the same regions in South America.

MÜllerian Mimicry

The second kind of mimicry—MÜllerian mimicry—is where one unpalatable creature resembles another. This form of mimicry is named after Fritz MÜller, who suggested the explanation now usually accepted, namely, that “Life is saved by a resemblance between the warning colours in any area, inasmuch as the education of young inexperienced enemies is facilitated, and insect life saved in the process.” “It is obvious,” writes Poulton (p. 328 of Essays on Evolution), “that the amount of learning and remembering, and consequently of injury and loss of life involved in these processes, are reduced when many species in one place possess the same aposematic colouring, instead of each exhibiting a different danger signal. . . . The precise statement of advantage was made by Mr Blakiston and Mr Alexander, of Tokio. ‘Let there be two species of insects equally distasteful to young birds, and let it be supposed that the birds would destroy the same number of individuals of each before they were educated to avoid them. Then if these insects are thoroughly mixed and become undistinguishable to the birds, a proportionate advantage accrues to each over its former state of existence. These proportionate advantages are inversely in the duplicate ratio of the respective percentages that would have survived without the mimicry.’”

This is rather a cumbrous method of saying that if there are in a locality a number of young birds, and each of these has to learn by experience which insects are edible and which are not, each will, if it learns by one example, devour one insect of any given pattern. Now, if two species of inedible insects have this pattern, they will between them lose only one member in the educating process of each bird, whereas if each species of insect had a colouration peculiar to itself, each species would lose a whole individual instead of half a one. There can be no doubt that such a livery of unpalatability is of some advantage to its possessors.

It has been shown experimentally that hand-reared young birds have to acquire their knowledge of flavours and colours by experiment.

It is well known that in many species the male and the female are not coloured alike. Such species are said to exhibit sexual dimorphism. In these cases it is usually the male that is more conspicuously coloured. Darwin felt that the theory of natural selection could not satisfactorily account for this phenomenon, so put forward the supplementary theory of sexual selection. On this hypothesis the females are supposed to be able to pick and choose their mates, and to select the most beautiful and ornamental ones, hence the greater showiness of these in most sexually dimorphic species. Wallace does not accept this theory. He thinks it unnecessary. He looks upon the brilliant colouring of the males as due to their superior vigour; moreover, he says that it is the hen that sits upon the eggs, and so requires a greater degree of protection than the male, and therefore natural selection has not permitted her to develop all the ornaments displayed by the cock. With the phenomenon of sexual dimorphism we shall deal at length in the next chapter.

Danger Signals

Dr Wallace recognizes yet another exception to the rule that animals are cryptically coloured. Many creatures possess on the body markings which tend to render them conspicuous rather than difficult to see. Where such markings occur on gregarious animals, Wallace believes that they have been evolved by natural selection, either to enable their possessors to recognize one another, or to act as a danger signal to their fellows. The white tail of the rabbit is believed by Wallace to serve as a danger signal. The first member of the company to espy the approaching foe takes to his heels, and, as he moves, his white tail catches the eye of his neighbour, who at once follows him, so that, in less time than it takes to tell, the whole company of rabbits is scampering towards the burrow, thanks to the white under-surface of the tail.

Even as Wallace out-Darwin’s Darwin, so does Mr Abbott Thayer, an American naturalist and artist, out-Wallace Wallace. That gentleman seems to be of opinion that all animals are cryptically or, as he calls it, concealingly or obliteratively coloured. Even those schemes of colour which have hitherto been called conspicuous are, he asserts, “purely and potently concealing” when looked at properly, that is to say, with the eye of the artist.

Lest it be thought unnecessary to criticize a hypothesis which appears to be based upon the assumption that animals see with the eye of the artist, we may say that Professor Poulton writes approvingly of Thayer’s theory. He frequently alludes to it in his Essays on Evolution, and he published an account of it in the issue of Nature, dated April 24, 1902. Moreover the hypothesis has been enunciated in such scientific journals as The Auk (1896) and The Year-Book of the Smithsonian Institution (1897).

Thayer asserts that all animals, or at any rate the great majority, including many that are usually supposed to be conspicuously coloured, are in reality obliteratively coloured—that is to say, coloured in such a way that the effects of light and shade are completely counteracted, with the result that they are invisible.

Obliterative Colouring

It is possible, says Mr Thayer, to almost obliterate a statue in a diffused light, by putting white paint on the surfaces in darkest shadow and dark paint on the most brightly lighted parts, all in due proportion. Now this is precisely what nature is supposed by Mr Thayer to have done for all her creatures.

It is well known that a great many animals, as for example the Indian black-buck and the hare, are coloured on the upper side and white below. This is called by Mr Thayer the principle of the gradation of colour. It runs, he declares, all through the animal world, and is “the main essential step toward making animals inconspicuous under the descending light of the sky.”

Animals, he contends, are not protectively coloured to look like clods or stumps or like surrounding objects, they are simply obliteratively coloured—coated, as it were, with invisible paint.

To quote from The Century Magazine (1908): “Whales, lions, wolves, deer, hares, mice; partridges, quails, sandpipers, larks, sparrows; frogs, snakes, fishes, lizards, crabs; grasshoppers, slugs, caterpillars—all these animals, and many thousands more, crawl, crouch, and swim about their business, hunting and eluding, under cover of this strange obliterative mask, the smooth and perfect balance between shades of colour and degrees of illumination.”

Nature having thus visually unsubstantialized the bodies of animals, so that, if seen at all, they look flat and ghostly, does not stop there. From solid-shaded bodies they have been converted, as it were, into flat cards or canvases, and, to complete the illusion of obliteration, pictures of the background—veritable pictures of the more or less distant landscape—have been painted on their canvases! Such in effect are the elaborate “markings of field and forest birds.”

Again he writes: “Brilliantly changeable or metallic colours are usually supposed to make the birds that wear them conspicuous, but nothing could be further from the truth. Iridescence is, indeed, one of the strongest factors of concealment. The quicksilver-like intershifting of many lights and colours, which the slightest motion generates on an iridescent surface, like the back of a bird or the wing of a butterfly, destroys the visibility of that wing or back as such and causes it to blend inextricably with the gleaming and scintillating labyrinthine-shadowed world of wind-swayed leaves and flowers.”

According to Thayer, the skunk, which for years has been an important item of the stock-in-trade of the advocates of the theory of warning colouration, is an excellent example of obliterative colouring, since its enemies are supposed to mistake for the sky-line the line of junction between the white fur of the back and the dark fur of the sides. Similarly the crocodiles are supposed to mistake a flamingo for the sky at sunrise or at sunset!

There is doubtless something in this theory of obliterative colouration.

Any one can see, by paying a visit to the South Kensington Museum, that an animal which is of a lighter colour below than above, is less conspicuous in a poor light than it would be were it uniformly coloured. There is then no doubt that this scheme of colour, which is so common in nature, has some protective value.

To this extent has Mr Thayer made a valuable contribution to zoological science. But when he informs us that obliterative colouring is a “universal attribute of animal life,” we feel sorely tempted to poke fun at him.

We would ask all those who believe in the universality of obliterative colouring to observe a flock of rooks wending their way to their dormitories at sunset.

Let us now pass on to the examination of the more orthodox theories of animal colouration.

Objections to the Theory of Cryptic Colouring

Before criticising the theory of cryptic colouring, we desire to state distinctly that we admit that, where other things are equal, it is of advantage to all creatures which hunt or which are preyed upon to be inconspicuous. If difficult to distinguish amid their natural surroundings, the former are likely to secure their prey readily, and the latter have a chance of escaping from their enemies. Our quarrel is with the theory of cryptic colouring as it is enunciated by many Neo-Darwinians, with the theory that every hue, every marking, every device displayed by an organism is of utility to the organism and has been directly developed by natural selection.

The extreme advocates of the theory of cryptic colouring have greatly exaggerated the degree in which animals are assimilated to their natural environment.

Fauna of Polar Regions

We grant that a great many creatures, which when seen in a menagerie appear very conspicuous, are the reverse of conspicuous when standing motionless amid their natural surroundings. As Beddard has pointed out, it is often not easy to find a sixpenny piece which has been dropped on the carpet, but the reason for this is, not that the coin is protectively coloured, but that any small object, no matter how coloured, is difficult to distinguish amid a variegated environment. The assumption of a white winter coat by many organisms that live in northern latitudes has been cited, again and again, as showing how important it is for an animal to be protectively coloured. If, it is urged, those creatures that live in lands which are covered in snow for half of the year have become white in winter by the action of natural selection in order to escape their foes, it is obviously of paramount importance to all creatures that they should be cryptically coloured. Popular books on natural history convey the impression that during winter the snow-clad, ice-bound Arctic regions are peopled by a fauna whose fur or hair rivals in whiteness the snowy mantle of the earth. The impression thus conveyed is misleading. It is true that an unusually large percentage of the animals that inhabit the polar regions are white in winter, but the majority of the creatures which dwell there do not assume the white garb of winter.

As the fauna of the polar regions is a small one, we are able to give lists of all the birds and mammals which dwell in the Arctic and the Antarctic regions. We have arranged these in in three columns. In the first are placed those creatures which are white throughout the year, in the third those that retain their colour through the winter, while the middle column contains those forms which change their colouring with the season.

ARCTIC FAUNA.

Mammals.

White.

Polar Bear.

Arctic Fox (some individuals).

White Whale or Beluga.

Changing with the Seasons.

Arctic Fox (most individuals).

Arctic Lemming.

Stoat.

Weasel.

Blue Hare.

Coloured.

Arctic Fox (sometimes).

Reindeer.

Musk-ox.

Glutton.

Moose.

Sable.

Seals.

Walrus.

Narhwal.

Greenland Whale.

Birds.

White.

Ivory Gull.

Snowy Owl.

Gyrfalcon.

Snow Goose.

Changing with the Seasons.

Black Guillemot.

Ptarmigans.

Snow Bunting (whitest in summer!)

Razorbill.

Little Auk (throat only becomes white).

Coloured.

Sea Eagle.

Greenland Redpoll (very pale).

All Arctic Geese and Ducks other than Snow Goose.

Raven.

Cormorant.

Brunnich’s Guillemot.

Puffin.

Fulmar Petrel.

Ross’s Gull.

Glaucous Gull (very pale).

Sandpipers.

ANTARCTIC FAUNA.

Mammals.

White.

Antarctic White Seal (Lobodon carcinophaga), in some cases.

Changing with the Seasons.

None.

Coloured.

Other Seals than Lobodon.

Whales.

Birds.

White.

Sheathbill.

Snowy Petrel.

Giant Petrel (some individuals).

Chick of Emperor Penguin.

Changing with the Seasons.

None.

Coloured.

Penguins.

Cormorant.

Skua Gull.

Giant Petrel (usually).

Other Petrels.

It will be observed that the third column contains the largest number of forms. It is thus evident that the whiteness of the Arctic and Antarctic faunas in winter has been greatly exaggerated.

The Arctic fox appears in all three columns, as the creature seems to fall into three races—a permanently white race, a permanently coloured race, and a seasonally dimorphic race.

Of the creatures set forth in the middle column of the above tables all are whiter in winter than in summer with the exception of the snow bunting, who sets at naught the theory of cryptic colouring by turning darker in winter! The same may be said of the Alpine chamois.

The advocates of the theory of protective colouring assert that the creatures which do not turn white in winter are strong and active animals which have no enemies to fear.

This contention is met by F. C. Selous as follows (African Nature Notes and Reminiscences, p. 9): “According to the experience of Arctic travellers, large numbers of young musk oxen are annually killed by wolves. . . . Nothing, I think, is more certain than that a far smaller percentage of so-called protectively coloured giraffes are killed annually by lions in Africa than of musk oxen by wolves in Arctic America.”

Another difficulty which confronts the Neo-Wallaceian school is that, ex hypothesi, the assumption of the white coat was gradual. Hence the change in the direction of whiteness cannot, in its first beginning, have been of perceptible utility to an organism. How then can natural selection have operated on it?

Pelagic Organisms

The transparency of pelagic organisms is frequently cited as exemplifying cryptic colouring. We all know that the common jelly-fish is as transparent as glass. Floating on the surface of the ocean are millions of tiny organisms, so transparent as to be invisible to the human eye. At first sight this certainly appears to be a remarkable case of protective colouring. Unfortunately, nearly all the more highly developed forms display conspicuous pigment (as in most jelly-fish) in some part of the body.

“An animal floating about in the sea,” writes Beddard, “perfectly transparent, but decked with dense black patches, of the size of saucers, would betray its whereabouts even to the least observant; if the observer were stimulated by hunger or fear, the conspicuousness would not be lessened. . . . Besides the internecine warfare which is continually going on amongst the smaller surface organisms, they are devoured wholesale by the larger pelagic fish, and by whales and other Cetacea. A whale, rushing through the water with open mouth and gulping down all before him, is not the least inconvenienced by the invisibility of the organisms devoured in such enormous quantities; nor do a solid phalanx of herring or mackerel stop to look carefully for their food: they take what comes in their way, and get plenty in spite of ‘protective absence of colouration.’

“If the transparency of the pelagic organisms be due entirely to natural selection, it is remarkable that there is so little modification in this direction among the species inhabiting the bottom at such depths as are accessible to the sun’s rays; the advantage gained by this transparency and consequent invisibility would be equally great. And yet this is not the case; the bulk of the bottom fauna of the coasts are brilliantly coloured animals, and those that show any protective colouring at all appear to be coloured so as to resemble stones or sea-weeds.”[7]

Before leaving the subject of marine animals, we may point out that the majority of the creatures that live in the everlasting blackness of the depths of the ocean display exceedingly conspicuous colouring, and this colouring seems to be constant. In such cases the colouring cannot be useful as such to its possessors. The same may be said of the colour of blood, or of the colouring of the internal tissues of all organisms. We must not lose sight of the fact that every organism, and every component part thereof, must of necessity be either of some colour or perfectly transparent. It seems to us that since the appearance of The Origin of Species zoologists have tended to exaggerate the importance of colouring to organisms; they frequently speak of it as though it were the one and only factor in the struggle for existence. It is on this account that they feel it incumbent upon them to find ingenious explanations for every piece of colouring displayed by every plant or animal.

Unimportance of Colour

The tendency to exaggerate the importance to an animal of its colouring is doubtless in large part due to the fact that many zoologists are content to study nature in museums rather than in the open. Some of those who observe organisms in their natural surroundings, especially in such favourable localities as the tropics, seem to be of opinion that natural selection has but little influence on the colouration of organisms.

Thus D. Dewar writes (Albany Review, 1907): “Eight years of bird-watching in India have convinced me that, so far as the struggle for existence is concerned, it matters not to a bird whether it be conspicuously or inconspicuously coloured, that it is not the necessity for protection against raptorial foes which determines the colouring of a species; in short, that the theory of protective colouration has but little application to the fowls of the air.”

Similarly, F. C. Selous writes, on page 13 of African Nature Notes and Reminiscences: “Having spent many years of my life in the constant pursuit of African game, I have certainly been afforded opportunities such as have been enjoyed by but few civilised men of becoming intimately acquainted with the habits and life-history of many species of animals living in that continent, and all that I have learned during my long experience as a hunter compels me to doubt the correctness of the now very generally accepted theories that all the wonderfully diversified colours of animals—the stripes of the zebra, the blotched coat of the giraffe, the spots of the bushbuck, the white face and the rump of the bontebok, to mention only a few—have been coloured either as means of protection from enemies or for the purpose of mutual recognition by animals of the same species in times of sudden alarm.”

So also G. A. B. Dewar—a very close observer of nature in England—writes, in The Faery Year: “Few theories in natural history have received more attention of late years than protective or aggressive colour, ‘mimicry,’ and harmony with environment. . . . To doubt this use of colour to animals seems like inviting back chaos in place of cosmos—for abandon the theory, and a world of colour is straightway void of purpose, a muddle of chance. So we all like the theory. Some, however, perceive plans to aid the wearer in every colour, tint, shade, and pattern. We may be sceptical of a good many of the cases they cite in support of colour aid, though attracted by the main idea.”

Writing of the commoner British butterflies, he says: “After a little practice, any man furnished with good eyesight can easily distinguish these butterflies—blues, coppers, small heaths, and meadow browns—from their perches; and so we may be sure that the small beast, bird, or insect of prey, with sense of colour or form, could also distinguish them. . . . Quite often, without even searching for them, I can see cabbage whites and other butterflies asleep on perches to which they by no means assimilate.” Mr G. A. B. Dewar suggests that the safety of the resting butterfly lies in “the position, the couch on high, . . . not the mask of colour or marking.”

Gadow on Coral Snakes

Two short visits to Southern Mexico sufficed to show Dr Hans Gadow that some of the commonly accepted explanations of colour phenomena are not the correct ones.

Thus writing of coral snakes, he says, on page 95 of Through Southern Mexico: “They are usually paraded as glaring instances of warning colouration, but I am not at all sure whether this is justifiable. Certainly these Elaps are most conspicuous and beautiful objects. Black and carmine or coral red, in alternate rings, are the favourite pattern; sometimes with narrow golden-yellow rings between them, as if to enhance the beautiful combination. But these snakes are inclined to be nocturnal in their habits, and, except when basking, spend most of their time under rotten stumps, in mouldy ground, or in ants’ nests in search of their prey, which must be very small, to judge from the size of the mouth.”

Dr Gadow goes on to show that although black and red are very strong contrasts in the day-time, the combination ceases to be effective in the dark. He suggests that red and black is a self-effacing rather than a warning pattern. He further points out that several kinds of harmless snakes have the same colouring and pattern. “There seems,” he says, “to be no reason why we should not call these cases of mimicry; and yet this is most likely a wrong interpretation, since such harmless snakes are also found in districts where the Elaps does not occur, not only in Mexico, but likewise in far-distant parts of the world, where neither elapines nor any other similarly coloured poisonous snakes exist. To interpret this as an instance of ‘warning colours’ in a perfectly harmless snake, which has no chance of mimicry, amounts in such cases to nonsense, and we have to look for a different explanation upon physiological and other grounds.”

It is, to say the least of it, significant that all the opposition to the theory of protective colouration comes from those who observe nature first hand, while the warmest supporters of the theory are cabinet naturalists and museum zoologists.

In the case of nocturnal creatures, as Dr H. Robinson very sagely points out (Knowledge, January 1909), the value for protective purposes of any given colouration must depend very largely on the state of the moon. “It was,” he writes, “a common experience in the South African War that on overcast or moonless nights the nearly black army great-coat made a picquet sentry invisible at a distance of a few feet. In strong moonlight this garb could be seen at a great distance, whereas a khaki pea jacket, useless on a dark night, answered the requirements of invisibility very well.” It is thus evident that the dark colour of the buffalo and sable antelope cannot be protective on both dark and moonlight nights.

The theory of protective colouration is based on the tacit assumption that beasts of prey rely on eyesight for finding their quarry. Raptorial birds certainly do use their eyes as the means of discovering their victims; but the great majority of predaceous mammals trust almost entirely to their power of smell as a means for tracking down their prey.

F. C. Selous Quoted

“Nothing,” writes F. C. Selous, on page 14 of African Nature Notes and Reminiscences, “is more certain than that all carnivorous animals hunt almost entirely by scent until they have closely approached their quarry, and usually by night, when all the animals on which they prey must look very much alike as far as colour is concerned.”

The herbivora—the quarry for the beast of prey—too, have a keen sense of smell, so that they trust their noses rather than their eyes for safety.

No observer of nature can have failed to remark how the least movement on the part of an animal will betray its whereabouts, even though in colouring it assimilates very closely to the environment. So long as the hare squats motionless in the furrow, it may remain unobserved, even though the sportsman be searching for it; but the least movement on its part at once attracts his eye. Thus, in order that protective colouration can be of use to its possessor, the latter must remain perfectly motionless. But, in tropical countries, where flies, gnats, etc., are a perfect scourge, no large animal is, when awake, motionless for ten seconds at a time. The tail is in constant motion, flicking off the flies that attempt to settle on the quadruped. The ears are used in a similar manner. Thus the so-called protective colouring of herbivora cannot afford them much protection. It is further worthy of note that the brush-like tip to the tail of many mammals is not of the same colour as the skin or fur. It is very frequently black. Thus we have the spectacle of a protectively coloured creature continually moving, as if to attract attention, almost the only part of its body that is not protectively coloured!

Sexual Dimorphism

Many species of birds display what is known as seasonal dimorphism, still more display sexual dimorphism.

Seasonally dimorphic birds very often assume a bright livery at the breeding season; this nuptial plumage is by no means invariably confined to the cock, so that we are brought face to face with the fact that some hen birds, that are normally inconspicuously coloured, become showy and easy to see at the nesting time, that is to say, precisely at the season when they would seem to be most in need of protection.

In the great majority of cases of sexual dimorphism among birds the cock is the more showily coloured. Now, if it be a matter of life-and-death importance to a bird to be protectively coloured, we should expect the showily coloured cock birds to be far less numerous than the dull-plumaged hens, since the former are, ex hypothesi, exposed to far greater danger than the inconspicuous hens. As a matter of fact, cock birds in practically all species appear to be at least as numerous as the hens. Nor can it be said that this is due to their more secretive habits. As a general rule, cock birds show themselves as readily as the hens; indeed, in the case of the familiar blackbird, the conspicuous cock is less retiring in his habits than the more sombre hen. It may, perhaps, be thought that the greater danger to which the sitting bird is exposed accounts for the fact that hens, notwithstanding their protective colouration, are not more numerous than the cocks. Unfortunately for the supposition, in many sexually dimorphic hens, as, for example, the paradise fly-catcher (Terpsiphone paradisi), the showy cock shares the burden of incubation equally with the hen.

It frequently happens that allied species of birds are found in neighbouring countries. The Indian robins, for example, fall into two species. The brown-backed robin (Thamnobia cambayensis) occurs north of Bombay, while the black-backed species (T. fulicata) is found south of Bombay. The hens of these two species are almost indistinguishable, but the cocks differ, in that one has a brown back, while the other’s back is glossy black. The Wallaceian theory of colouration seems quite unable to explain this phenomenon—the splitting up of a genus into local species—which is continually met with in nature. Equally inimical to the theory of protective colouration is the existence, side by side, of species which obtain their living in much the same manner. On every Indian lake three different species of kingfisher pursue their profession cheek by jowl; one of these—Ceryle rudis—is speckled black and white, like a Hamburg fowl; the second is the kingfisher we know in England; and the third is the magnificent white-breasted species—Halcyon smyrnensis—a bright-blue bird with a reddish head and a white wing bar. It is obvious that all three of these diversely plumaged species cannot be protectively coloured. It may perhaps be objected that the piscatorial methods of these kingfishers differ in detail. We admit that this is the case, but would maintain, at the same time, that these comparatively slight differences in habit do not account for the very striking differences in plumage. We may also cite the yellow and pied wagtails of our own country, which may be seen feeding in the same meadows. Most familiar and striking of all is the everyday sight of a blackbird and thrush plying their respective avocations within a few yards of each other on the same lawn, differently coloured though they be.

Another weighty objection to the generally accepted theory of protective colouration is that some of the creatures which assimilate most closely to their environment are those which appear to be the least in need of such protection.

Precis Artexia

The butterfly Precis artexia, writes F. C. Selous, “is only found in shady forests, is seldom seen flying until disturbed, and always sits on the ground amongst dead leaves. Though handsomely coloured on the upper side, when its wings are closed it closely resembles a dead leaf. It has a little tail on the lower wing, which looks exactly like the stalk of a leaf, and from this tail a dark-brown line runs through both wings (which on the under side are light brown) to the apex of the upper wing. One would naturally be inclined to look upon this wonderful resemblance to a dead leaf in a butterfly sitting with closed wings on the ground amongst real dead leaves as a remarkable instance of protective form and colouration. And of course it may be that this is the correct explanation. But what enemy is this butterfly protected against? Upon hundreds of different occasions I have ridden and walked through forests where Precis artexia was numerous, and I have caught and preserved many specimens of these butterflies, but never once did I see a bird attempting to catch one of them. Indeed, birds of all kinds were scarce in the forests where these insects were to be found.”

Similarly D. Dewar writes (Albany Review, 1907): “If a naturalist be asked to cite a perfect example of protective colouring, he will, as likely as not, name the sand grouse (Pteroclurus exustus). This species dwells in open, dry, sandy country, and its dull brownish-buff plumage, with its soft dark bars, assimilates so closely to the sandy environment as to make the bird, when at rest, practically invisible, at any rate to the human eye. Unfortunately for the theory, this bird stands less in need of protective colouration than any other, for it has wonderful powers of flight. Even a trained falcon is unable to catch it, because it can fly upwards in a straight line as though it were ascending an inclined plane, with the result that the pursuing hawk is never able to get above it to strike.”

Striped Caterpillars

Lord Avebury, who is a typical Wallaceian, points out the connection that exists between longitudinal stripes on caterpillars and the habit of feeding either on grass or low-growing plants among grass. The inference, of course, is that birds mistake these caterpillars for leaves, or, at any rate, fail to observe them when feeding, not only because they are green in colour, but because their longitudinal stripes look like the parallel veins on the blades of grass. But the butterflies of the family SatyridÆ, as Beddard points out, all possess striped larvÆ, and these feed chiefly by night, when neither their colouring nor marking is visible, while during the day many of them lie up under stones; other caterpillars of this family feed inside the stems of plants. “Now,” writes Beddard (Animal Colouration, p. 101), “in these cases the colour obviously does not matter: if, therefore, the longitudinal striping is kept up by constant selection on account of its utility, and has no other signification, we might expect that in these two species (Hipparchia semele and Œnis), and in others with similar habits, the cessation of natural selection would have permitted the high standard required in the other cases to be lowered—perhaps, even, as has been suggested in the case of cave animals, the colours being useless to their possessors, might have disappeared altogether—but they have not.”

Many exceedingly conspicuous birds—as, for example all the crow-tribe, the egrets, the kingfishers—flourish in spite of their showy plumage. Such creatures, while scarcely constituting a valid objection to the theory of protective colouration, serve to show that protective colouring is not a necessity. An animal otherwise able to take care of itself can afford to dispense with cryptic colouration. “An ounce of good solid pugnacity is a more effective weapon in the struggle for existence than many pounds of protective colouration.”

There used to live in the gardens of the Zoological Society of London a black cat belonging to the manager of one of the restaurants. This animal used to catch birds on the lawn. We believe that not even Mr Thayer will maintain that a black cat is cryptically coloured when stalking on a well-watered lawn! Nevertheless the nigritude of that cat did not prevent it securing a meal.

Colours of Eggs

The case of birds’ eggs furnish an excellent example of the lengths to which Wallace and his followers have pushed the theory of protective colouration.

D. Dewar maintains that it is possible to divide birds’ eggs that are coloured, as opposed to those that are white, into two classes—those which are protectively coloured and those which are not. The former class includes all those which are laid in shingle or on the bare ground, as, for example, the eggs of the ring-plover and the lap-wing.[8] He maintains that the variously coloured and speckled eggs that are laid in cup-shaped nests are not protectively coloured at all; he declares that they are usually very conspicuous when in the nest, and, moreover, it would be futile for them to be cryptically coloured, for a bird or lizard that habitually sucks eggs will examine carefully the interior of each nest it discovers.

Needless to say, this view does not appeal to the so-called Neo-Darwinians. Wallace writes, on page 215 of Darwinism: “The beautiful blue or greenish eggs of the hedge-sparrow, the song-thrush, the blackbird, and the lesser redpole seem at first sight especially calculated to attract attention, but it is very doubtful whether they are really so conspicuous when seen at a little distance among their usual surroundings. For the nests of these birds are either in evergreen, or holly, or ivy, or surrounded by the delicate green tints of early spring vegetation, and may thus harmonise very well with the colours around them. The great majority of the eggs of our smaller birds are so spotted or streaked with brown or black on variously tinted grounds that, when lying in the shadow of the nest and surrounded by the many colours and tints of bark and moss, of purple buds and tender green or yellow foliage, with all the complex glittering lights and mottled shades produced among these by the spring sunshine and sparkling rain-drops, they must have quite a different aspect from that which they possess when we observe them torn from their natural surroundings.”

The obvious comment on this is that it is very fine and poetic English, but it is not science. It is futile to deny what should be obvious to every field naturalist, namely, that the majority of eggs laid in open nests are most conspicuous.

D. Dewar thus summarises the main facts which show that eggs in nests (as opposed to those laid on the bare ground) are not protectively coloured:—

“1. Allied species of birds, even though their nesting habits are very different, as a rule lay similarly coloured eggs.

“2. Eggs laid in domed nests certainly do not need protective colouring, yet many of these are coloured.

“3. The same is true of many eggs laid in holes in trees or in buildings.

“4. The protective resemblances of eggs which are laid in the open are apparent to everyone, which certainly is not true of those deposited in nests.

“5. Many birds lay eggs which exhibit very great variations.

“6. Some birds lay eggs of different types, and these sometimes differ from one another so greatly that it is difficult to believe that they could have been laid by the same species.”[9]

7. It not infrequently happens that one species lays in the disused nest of another, and the eggs of the latter are often very different in colouring from those of the former.

We have up to the present considered the theory of general cryptic colouration, which declares that the majority of creatures are so coloured as to be inconspicuous. We have still to deal with the hypothesis of special cryptic colouring.

Certain animals look, when resting, very like an inanimate object, such as a dead leaf or a twig. This resemblance is said to be the result of natural selection, since it enables its possessors to escape destruction; they are seen, but mistaken for something else.

The classical examples of this kind of protective colouring are furnished by the Kallimas or leaf-butterflies, which display an extraordinary resemblance to dead leaves.

Other examples are the stick-insects and the lappet moth, which looks like a bunch of dry leaves. It is needless to multiply instances. In every work on animal colouration numbers of such cases are cited.

We may grant that in some cases, at any rate, the resemblance is of value to its possessor, in that it deceives predatory creatures. But it does not follow from this that the likeness has originated through the action of natural selection. In order that there can be selection there must be varying degrees of a tolerable resemblance to select from. How did the initial similarity arise? This is a matter upon which Wallaceians are silent. As Poulton truly says, in discussing the degree of protection afforded by such resemblances, we tacitly endow animals with senses exactly similar to our own. Are we justified in so doing? Most certainly not in the case of the invertebrate animals, especially as regards the arthropods, of which the eyes are constructed very differently from those of human beings.

D. Dewar has often seen a toad shoot out its tongue and touch a lighted cigarette end, apparently mistaking it for an insect. Similarly, he has again and again induced a gecko lizard to chase and try to swallow a piece of black cotton, one end of which was rolled up into a ball. It is only necessary to take hold of the unrolled end of the cotton and place the rolled-up end a few inches from the lizard, and gradually draw it away in order to induce the lizard to attempt to seize it.

Eyesight of Birds

It would therefore seem that all these elaborate “protective” devices are unnecessary refinements if regarded as a protection against invertebrate, reptilian, and amphibian foes. Birds, on the other hand, appear to have exceedingly sharp eyesight, so that in order to deceive them the resemblance requires to be very close. Indeed, as regards those birds which systematically hunt for their prey among leaves and grass, it seems doubtful whether the alleged “protective” resemblances of caterpillars to twigs, etc., are sufficient to be of much use to them. Thus Beddard writes (on page 91 of Animal Colouration): “Judging of birds by our own standard—which is the way in which nearly all the problems relating to colour have been approached—does it seem likely that we should fail to see a caterpillar, perhaps as long or longer than the arm, of an obviously different texture from the branches, and displaying in many cases through its semi-transparent skin the pulsation of the heart, for which we were particularly searching?”

Now, birds certainly feed very largely on caterpillars, while they are but rarely seen to eat butterflies. If, therefore, the aim and object of these special resemblances is the protection of the species, we should expect to see them in a nearly perfect state in caterpillars on which birds feed very largely, and poorly developed in butterflies, which do not appear to be greatly preyed upon by birds, but have to fear chiefly the comparatively dull-eyed lizards and mammals, of which the latter hunt mainly by scent. As a matter of fact, the most striking cases of resemblance to inanimate objects are seen among butterflies, which seem to stand least in need of them.

We have already cited the case of the butterfly Precis artexia. Even more marked does the unnecessary elaboration of the likeness seem to be in the Kallima butterflies.

The Theory of Warning Colouration

All biologists admit that there exist some organisms which are not coloured so as to be inconspicuous. Indeed, the colouring of certain species is such as to render them particularly conspicuous. Such species are said to be warningly coloured. They are supposed to be inedible, or to have powerful stings or other weapons of defence, or to resemble in appearance organisms which are thus protected. In the first two cases they are said to be warningly coloured, and in the last they are cited as examples of protective mimicry. With the theory of mimicry we shall deal shortly. We must first discuss the hypothesis of warning colouration.

When animals are unpalatable, or when they possess a sting or poison-fangs, it is, to use the words of Wallace, “important that they should not be mistaken for defenceless or eatable species of the same class or order, since in that case they might suffer injury, or even death, before their enemies discovered the danger or the uselessness of the attack. They require some signal or danger-flag which shall serve as a warning to would-be enemies not to attack them, and they have usually obtained this in the form of conspicuous or brilliant colouration, very distinct from the protective tints of the defenceless animals allied to them” (Darwinism, page 232).

Examples of Warning Colouration

For examples of so-called warningly coloured animals, we may refer the reader to Wallace’s Darwinism, Poulton’s Essays on Evolution, or Beddard’s Animal Colouration. An instance familiar to all is our English ladybird. “Ladybirds,” says Wallace, “are another uneatable group, and their conspicuous and singularly spotted bodies serve to distinguish them at a glance from all other beetles.”

In order to establish the theory of warning colouration, it is necessary to prove that all, or the great majority of conspicuously-coloured organisms, are either unpalatable or mimic unpalatable forms. If this be so, we are able to understand that the possession of gaudy colouring may be of advantage to the individual. But even if this be satisfactorily proved, we must bear in mind that it does not necessarily follow that these warning colours can be accounted for on the theory of natural selection. For, in order to explain the existence of any organ by the action of natural selection, we must be able to demonstrate the utility, not only of the perfected organ, but of the organ at its very beginning, and at each subsequent stage of development. This, as we shall show, is precisely what the Neo-Darwinians are unable to do. We shall have no difficulty in proving that it would be more advantageous even to a highly nauseous creature to have remained inconspicuously coloured rather than to have gradually become more and more conspicuous.

In the first place, let us briefly examine the evidence on which rests the assertion that all gaudily-coloured insects, etc., are unpalatable, or possess stings, or mimic forms which are thus armed.

In England wasps, bees, and ladybirds are familiar examples of conspicuous insects.

The banded black and yellow pattern of the common wasp and the humble bee are regarded as advertisements or danger signals of the powerful sting.

The red-coat with its black spots is similarly believed to be a warning that the ladybird is not fit to be eaten.

Caterpillars are usually coloured grey or brown, so as to be inconspicuous; but numerous exceptions occur which are brightly coloured, and of these individuals many have been experimentally proved to be objectionable as food to most insect-eating animals, being either protected by an unpleasant taste, or covered with hairs or spines.

Familiar cases are those of the abundant and conspicuous black and yellow mottled caterpillars of the European Buff-tip Moth (PygÆra bucephala), which are much disliked by birds; and the gaily—coloured Vapourer Moth caterpillar (Orgyia antiqua), with its conspicuous tufts of hair. Readers will remember that a few years back these caterpillars were a perfect plague in London, in spite of the abundance of sparrows, which feed freely on smooth green and brown caterpillars.

Oft-cited examples of warning colouration, are the three great groups of mainly tropical butterflies—the HeliconidÆ of America, the AcrÆidÆ of Africa, and the DanainÆ found all over the world. In all of these the sexes are alike. They are, every one, strikingly coloured, displaying patterns of black and red, chestnut, yellow, or white. In most butterflies the lower surface of the wings is of a quiet hue, in order to render the organism inconspicuous when at rest, but in these warningly coloured groups the under surface of the wings is as gaudy as the upper surface. Their flight is slow. They are tough, and exhale a characteristic odour.

Belt showed that, in Nicaragua, birds, dragonflies, and lizards seem to avoid the Heliconine butterflies, as the wings of these last are not found lying about in places where insectivorous creatures feed, whereas wings of the edible forms are to be found. Moreover, a Capuchin monkey, kept by Belt, always refused to eat Heliconine butterflies.

Finn investigated the palatability of a number of Indian insects. He found that most of the birds with which he experimented objected to the Danaine butterflies; but they disliked still more intensely two butterflies belonging to groups not universally protected—a swallowtail (Papilio aristolochiÆ) and a white (Delias eucharis).

Finn further experimented with the tree-shrew or Tupaia (Tupaia ellioti), which feeds largely on insects. He found that this creature refused most emphatically all these warningly-coloured butterflies. It would under no circumstances eat the DanainÆ, whereas the birds would do so if no more palatable insects were offered to them at the time.

Colonel A. Alcock found that a tame Himalayan bear indignantly refused to eat a locust (Aularches militaris) gaily coloured with black, red, and yellow, and exhaling an unpleasant-smelling froth; but this bear readily devoured ordinary brown or green species.

Among cold-blooded vertebrates the common European salamander, with its bright black and yellow markings, is a striking example of warning colouration; its skin exudes, on pressure, a very poisonous secretion.

Colonel A. Alcock has described a small siluroid sea-fish, brightly banded with black and yellow, and armed with poison spines.

A well-known Indian poisonous snake, the banded Krait (Bungarus coeruleus), is conspicuously barred with wide bands of black and yellow; and in South America there occur numerous species of coral snakes, in which red is added to these conspicuous colours.

The only known poisonous lizard—the Heloderm of Mexico—is conspicuously blotched with black and salmon-colour.

Among birds, no instances of warning colouration have been recorded, though Professor Poulton has suggested that possibly the striking and contrasted tints of many tropical species may be due to this cause. The suggestion is an ingenious one, but is at present totally unsupported by evidence.

The skunks are often cited as an excellent example of warning colouration among mammals. Skunks are most conspicuously arrayed in black and white—the latter above, not below, as is usual—and have bushy tails, which they carry erect. Although less powerful and ferocious than other members of the weasel family, to which they belong, skunks are notoriously protected by their abundant secretion of a very fetid liquid.

For further examples of warning colouration we would refer the reader to Beddard’s illuminating book, entitled Animal Colouration.

It should be noticed that in all the cases which we have cited the colouration is not only conspicuous, but is found in both sexes, whereas in many undefended animals the male may be just as strikingly coloured, but the female is not.

We may take it as proved that there is a very general relation between gaudy colouring and inedibility, or rather unpalatability, among insects. It may safely be said that any species of insect which lives, either as an adult or as a larva, in the open will perish in the struggle for existence if, being conspicuously coloured, it is neither inedible nor armed with a weapon such as sting, nor provided with a thick cuticle, nor resembles in appearance some creature which is protected.

Warning Colouring a Drawback

But from this it is not legitimate to conclude, as Neo-Darwinians do, that these brilliant colours have been slowly brought into being by natural selection.

Why should any creature, having by the “luck” of variation and heredity acquired some quality—be it strength, pugnacity, sting, or unpleasant taste—which renders it comparatively immune from persecution, proceed to advertise the fact by assuming a gaudy or striking colour? It would surely be better for such an organism to remain inconspicuous. By becoming showy it is visible to every young bird who, not having yet learned that the creature in question is unfit for food, seizes and perhaps kills it. It is true that the young bird vows that never again will it touch another such organism. But of what avail to the dying example of warning colouration is the resolution of the young bird? Moreover, the organism in question, by being conspicuous, also advertises itself to those few enemies which will eat it. There are always, as Professor Poulton justly remarks, animals which are enterprising enough to take advantage of prey which has at least the advantage of being easily seen and caught.

Conspicuous Animals Attacked

It is possible to cite cases where animals, notwithstanding the fact that they possess natural defences, become the prey of others in some exceptional cases.

The salamander can be eaten with comparative impunity by the toad, a creature very likely to meet with it.

The toad itself may be eaten; Finn saw the Indian toad (Bufo melanostictus) eat another of its own kind. He further observed that the Indian water-snake (Tropidonotus piscator) and the “Crow pheasant” cuckoo (Centropus sinensis), in the free state, and the Indian Roller (Coracias indica) and the Pied Hornbill (Anthracoceros), in captivity, eat the warningly-coloured toad. On the other hand, a captive Racket-tailed drongo rejected toads when offered to it. The common cuckoo is well known to feed on hairy and “warningly-coloured” caterpillars.

Finn has also seen the glossy cuckoo in Zanzibar devouring black-and-yellow caterpillars. Moreover, in America crows are found to select deliberately highly polished and strongly flavoured beetles. Yet again, wasps are preyed upon by bee-eaters, and also eaten by our common toad. In India, Finn found, by many experiments, that the common garden lizard, or “bloodsucker” (Calotes versicolor), would eat, both in captivity and in freedom, all “warningly-coloured” butterflies, not only the DanainÆ, but even Delias eucharis and the pre-eminently nauseous Papilio aristolochiÆ. That this reptile is a great enemy to butterflies is rendered probable by the frequent occurrence of specimens of these insects with its semicircular bites in their wings.

Further, Finn found that bulbuls, the commonest garden birds in India, ate the DanainÆ readily in captivity, even when other butterflies could be had, which was not the case with most other birds. Bulbuls did, however, usually refuse the Delias and Papilio mentioned above.

The Skunk is preyed upon in America by the Eagle-owl (Bubo virginianus) and the Puma.

Thus, animals provided with natural defences are not immune from attack.

Hence natural selection cannot have encouraged the survival of individuals which displayed a conspicuous colour, for the sake of the “warning.”

We must not forget that many creatures armed with powerful weapons possess the unobtrusive drab, brown, or green colouration which is associated with concealment from foes.

There can be little doubt that, but for the fact that the hive-bee can inflict a sting more severe than that of the wasp, this useful insect would have been cited as a case of a protectively coloured creature. Notwithstanding its sober brown colouring, the hive-bee is recognised and avoided.

Professor Poulton records that the dull inconspicuous caterpillar of the moth (MÆnia typica) is rejected by reptiles. It must be admitted, however, that these cases among insects are very rare.

The smooth newt (Molge vulgaris), a relation of the salamander, is protected by a poisonous skin; nevertheless the creature has a dark brown back and spends most of its time on land. Its black-spotted, yellow under-surface may have some protective value in the water. Neither the pike nor the common European water-tortoise will eat this newt.

Toads are nearly all very inconspicuous; nevertheless they are well protected by the acrid secretion from the skin glands; moreover, they are both recognised and avoided by those predacious creatures to whom they are distasteful. Hawks, although as a rule plainly coloured, are certainly recognised by all other birds. It would seem, therefore, that “warning colours,” like the similar striking hues of many domestic animals, are incidental attributes. It has been possible for their owners to develop them, because for the most part let alone.

Eisig, long ago, pointed out that the brightly coloured pigment in the skin of these warningly coloured insects is in certain cases of an excretory nature. Therefore the inference which should be drawn is, as Beddard points out on page 173 of his Animal Colouration, “that the brilliant colours (i.e. the abundant secretion of pigment) have caused the inedibility of the species, rather than that the inedibility has necessitated the production of bright colours as an advertisement.” In other words, Neo-Darwinians put the cart before the horse!

BOURU FRIAR-BIRD

Like most of the group to which it belongs, this honey-eater (Tropidorhynchus bouruensis) is a soberly coloured bird, but is noisy, active, and aggressive.

BOURU ORIOLE

This “mimicking” oriole (Oriolus bouruensis) is of the same tone of colour as its supposed model the Friar-bird of the same island.

In some cases these brilliantly coloured insects may be survivals of an age in which there were no birds. When these came into being and began to prey upon insects, the conspicuously coloured species which were not inedible or very unpalatable would soon become extinct, while those that were inedible would survive as warningly-coloured insects. In other cases it is not improbable that these warningly-coloured creatures have arisen by mutations from more soberly-hued insects. It is conceivable that every now and again a mutation occurs which renders its possessor conspicuous. This will result in the early destruction of these aberrant individuals unless their newly-acquired gaudiness is either correlated with, or the result of, distastefulness.

Aposematic Sounds

In the case of warning colouration, the Neo-Darwinians have, as usual, pursued their theory to absurd lengths. Professor Poulton, for example, extends it to sounds and attitudes. “Sound,” he writes, on page 324 of Essays on Evolution, “may be employed as an Aposematic character, as in the hiss of some snakes and some lizards. Certain poisonous snakes when disturbed produce by an entirely different method a far-reaching sound not unlike the hiss. Thus the rattle-snake (Crotalus) of America rapidly vibrates the series of dry, horny, cuticular cells, movably articulated to each other and to the end of the tail. The stage through which the character probably arose is witnessed in another genus which vibrates its tail among dry leaves, and thus produces a warning sound. The deadly little Indian snake (Echis carinata) (‘the Kuppa’) makes a penetrating swishing sound by writhing the coils of its body one over the other. Special rows of the lateral scales are provided with serrated keels which cause the sound when they are rubbed against each other. Large birds, when attacked, often adopt a threatening attitude, accompanied by an intimidating sound which usually suggests more or less closely the hiss of a serpent, and thus includes an element of mimicry. . . . The cobra warns an intruder chiefly by attitude and by the broadening of its flattened neck, the effect being heightened in some species by the ‘spectacles.’ In such cases we often witness a combination of cryptic and Aposematic methods, the animal being concealed until disturbed, when it instantly assumes a warning attitude.

“The benefit of such intimidating attitudes is clear: a venomous snake gains far more advantage by terrifying than by killing an animal it cannot eat. By striking, the serpent temporarily loses its poison, and with this a reserve of defence. Furthermore, the poison does not cause immediate death, and the enemy would have time to injure or destroy the snake.”

Intimidating Attitudes

At first sight this reasoning may seem very convincing. But consider for a moment the process by which the hiss originated and gradually increased by natural selection. We must suppose that the rattle-snake was formerly incapable of making any sound. One day a variety appeared in which the skin was slightly hardened, so that when the creature moved its body rapidly there issued a slight sound. This must have caused an enemy to refrain from attack; it thus lived to transmit this peculiarity to its offspring, and those which made more noise than their ancestors escaped, while those that made less succumbed to their enemies. For ourselves, we find it quite impossible to believe that the rattle was thus gradually evolved by means of natural selection. Indeed, we are inclined to think that neither the hiss of the cobra nor its “intimidating attitude” has any terrifying effect on its adversary. In the case of the cobra we are able to cite positive evidence that dogs and cattle show no alarm at the attitude.

“Dogs,” writes D. Dewar of this display, “regard it as a huge joke. Of this I have satisfied myself again and again, for when out coursing at Muttra we frequently came across cobras, which the dogs used invariably to chase, and we sometimes had great difficulty in keeping the dogs off, since they seemed to be unaware that the creature was venomous.”

Colonel Cunningham writes, on page 347 of Some Indian Friends and Acquaintances: “Sporting dogs are very apt to come to grief where cobras abound, as there is something very alluring to them in the sight of a large snake when it sits up nodding and snarling; and it is often difficult to come up in time to prevent the occurrence of irreparable mischief.”

Colonel Cunningham also states that many ruminants have a great animosity to snakes, and are prone to attack any that they may come across.

We may therefore well be sceptical as to the value of intimidating attitudes to those creatures which are in the habit of striking them.

Mimicry

In a work of this kind it is neither possible nor necessary to consider in great detail the mass of evidence which has been advanced in favour of the theory of mimetic resemblance.

Chapters vii. and viii. of Professor Poulton’s Essays on Evolution contain an up-to-date statement of the facts in favour of the theory. Professor Poulton believes that in all cases mimetic resemblance is the result of the action of natural selection.

He admits that there is no direct evidence in its favour, but asserts that “the facts of the cosmos, so far as we know them, are consistent with the theory, and none of them inconsistent with it” (page 271).

Theory of Protective Mimicry

We are not at all sure that no facts are against the theory of protective mimicry. We shall presently set forth some which to us seem, if not actually inconsistent with the theory, at least to point to the conclusion that the phenomenon may be explained otherwise than as a product of natural selection.

Evidence for the Theory

Let us first briefly state the case for the theory of protective mimicry.

1. It is asserted that the mimicking species and that which is mimicked are often not nearly related. For example, the unpalatable larva of the Cinnabar Moth (Euchelia jacobaeÆ) is said to mimic a wasp, because it has black and yellow rings round its body.

“The conclusion which emerges most clearly,” writes Poulton (p. 232), “is the entire independence of zoological affinity exhibited by these resemblances.” This is supposed to be proof that Darwin was wrong when he asserted that the original likeness was due to affinity. Says Poulton: “The preservation of an original likeness due to affinity undoubtedly explains certain cases of mimicry, but we cannot appeal to this principle in the most remarkable instances.”

2. It is asserted that species which are mimicked are invariably either armed with a sting, well defended, or unpalatable, so that it is against the interest of insectivorous creatures to attack them. It is further asserted that the species imitated are “even more unpalatable than the generality of their order.”

3. It is pointed out that the most distasteful groups of butterflies—the DanaidÆ, the AcrÆinÆ, the IthomiinÆ, and the HeliconinÆ—consist of large numbers of species which closely resemble one another. This is said to be due to MÜllerian mimicry. Mayer states that in South America there are 450 species of inedible IthomiinÆ which display only 15 distinct colours, while the 200 species of Papilio, which are edible, exhibit 36 distinct colours. Nevertheless, he says, there is no lack of individual variability among the former hence their conservatism as regards colour cannot be attributed to their having but little tendency to vary.

4. It is asserted that although in many cases the mimetic resemblances extend to the minutest detail, nevertheless they are not accompanied by any changes in the mimetic species except such as assist in the production or strengthening of a superficial likeness.

Pictures illustrating such cases of mimicry are figured on pp. 241, 247, and 251 of Wallace’s Darwinism (1890 edition).

5. It is stated that mimetic resemblance is not confined to colour, but extends to pattern, form, attitude, and movement; that deep-seated organs are affected when the superficial resemblance is intensified, but not otherwise. Poulton cites Clytus arietis, the “wasp-beetle,” as an example of this.

6. It is asserted that mimetic resemblances are produced in the most diverse ways; that the modes whereby the similarity in appearance is brought about are varied, but the result is uniform.

“A lepidopterous insect,” writes Poulton (p. 251), “requires above all to gain transparent wings, and this, in the most striking cases that have been studied, is produced by the loose attachment of the scales, so that they easily and rapidly fall off and leave the wing bare except for a marginal line and along the veins (Hemaris, Trochilium).”

7. It is alleged that the imitator and imitated are always found in the same locality. If they did not do so no advantage would be derived from the resemblance. It is further alleged that where the mimicking species is edible it is invariably less abundant where it occurs than the species it imitates.

8. It is pointed out that it sometimes happens that where in the mimic the sexes differ in appearance, the male copies one species, the female quite a different one. This is said to be because the deception would be liable to be detected if the mimicking species became common relatively to that which is imitated. “We therefore find that two or more models are mimicked by the same species” (Essays on Evolution, p. 372).

Occasionally the female mimics two other species, i.e. she occurs in two forms, each like a different species.

It sometimes happens that the female alone mimics. This is said by Wallace to be due to her greater need of protection. When she is laden with eggs her flight is slow, and therefore she requires a special degree of protection.

9. It is said that in some species we find a non-mimetic ancestor preserved on islands where the struggle for existence is less severe, while on the adjacent continent mimicry has been developed.

10. It is alleged that in the cases where moths resemble butterflies the former are either as diurnal as the butterflies or are species which “readily fly by day when disturbed.”

11. It is asserted that some seasonally dimorphic forms are examples of mimicry only in one state, in the form that comes into being at the time when the struggle for existence is most severe; that is to say, in the dry season, in Africa, when insect life is far less abundant than in the rainy season.

In other cases the mimicry of the dry-weather form is said to be far more perfect.

Instances of this phenomenon are set forth in Professor Poulton’s Essays on Evolution.

Alternative Theories

It will be observed that we have quoted very largely from Professor Poulton’s work. Our reason for so doing is that he appears to be the most prominent advocate of the theory of protective mimicry, and his work, which was published in 1908, may be taken as the latest Neo-Darwinian pronouncement on the subject.

Hence if we can show, as we believe we can, that his arguments are not sound, we may take it that we have demonstrated that the theory in its present form is untenable.

It is worthy of notice that Professor Poulton sets forth three other suggestions which have been proposed as substitutes for natural selection as an explanation of the phenomena of mimicry.

The first is the theory of External Causes, namely, that the resemblance is due to some external cause, such as food or climate.

The second is the theory of Internal Causes, which states that mimetic resemblance is due to internal developmental causes.

The third is the suggestion that sexual selection has caused the origin of these resemblances.

He then proceeds to demolish these to his own satisfaction, and adds triumphantly, “The conclusion appears inevitable that under no theory, except natural selection, do the various resemblances of animals to their organic and inorganic environments fall together into a natural arrangement and receive a common explanation” (p. 228).

To reasoning of this description there is an obvious reply. Even if it be granted that the alternatives to the theory of natural selection as set forth by Professor Poulton are untenable, it does not follow that natural selection affords an adequate explanation. If A, B, C and D are charged with theft and the prosecutor proves that neither A nor B nor C committed the theft, this will not suffice to secure the conviction of D. It is quite possible that a fifth person, E, may be the culprit.

Much of the popularity of the theory of natural selection is due to the fact that biologists have not yet been able to discover a substitute for it.

It seems to us that the proper method of making progress in science is not to bolster up natural selection by ingenious speculations, but to look around for other hitherto undiscovered causes.

KING-CROW OR DRONGO

This very conspicuous black bird (Dicrurus ater), ranging from Africa to China, is a striking feature of the landscape wherever it occurs.

DRONGO-CUCKOO

The fork of the tail in this bird is unique among cuckoos, but is nevertheless much less developed than in the supposed model, and may be an adaptation for evolution in flight, as such tails usually appear to be.

Objections to the Theory that the so-called Cases of Mimicry owe their Origin to Natural Selection

It is obvious that for one creature to resemble another can be of little or no benefit to either until the resemblance is tolerably close. It is, therefore, insufficient to prove the utility of the perfected resemblance. We may readily grant this and yet maintain that the origin of the resemblance cannot be due to the action of natural selection.

The Drongo-cuckoo (Surniculus lugubris) displays so great a likeness to the King Crow (Dicrurus ater) that it is frequently held up by Neo-Darwinians as an excellent example of mimicry among birds. But D. Dewar writes, on page 204 of Birds of the Plains: “I do not pretend to know the colour of the last common ancestor of all the cuckoos, but I do not believe that the colour was black. What then caused Surniculus lugubris to become black and assume a king-crow-like tail?

“A black feather or two, even if coupled with some lengthening of the tail, would in no way assist the cuckoo in placing its egg in the drongo’s nest. Suppose an ass were to borrow the caudal appendage of the king of the forest, pin it on behind him, and then advance among his fellows with loud brays, would any donkey of average intelligence be misled by the feeble attempt at disguise? I think not. Much less would a king-crow be deceived by a few black feathers in the plumage of a cuckoo. I do not believe that natural selection has any direct connection with the nigritude of the drongo-cuckoo.”

Darwin was fully alive to this difficulty when he wrote: “As some writers have felt much difficulty in understanding how the first step in the process of mimicry could have been effected through natural selection, it may be well to remark that the process probably commenced long ago between forms not widely dissimilar in colour” (Descent of Man, 10th Ed., p. 324). Such a statement is of course quite inconsistent with the Neo-Darwinian position. “The conclusion which emerges most clearly,” writes Poulton (Essays on Evolution, p. 232), “is the entire independence of zoological affinity exhibited by these resemblances; and one of the rare cases in which Darwin’s insight into a biological problem did not lead him right was when he suggested that a former closer relationship may help us to a general understanding of the origin of mimicry. The preservation of an original likeness due to affinity undoubtedly explains certain cases of mimicry, but we cannot appeal to this principle in the most remarkable instances.”

It is unnecessary to labour this point. It is surely evident to everyone with average intelligence that, until the resemblance between two forms has advanced a considerable way, the likeness cannot be of utility to either, or at any rate of sufficient utility to give its possessor a survival advantage in the struggle for existence. Until it reaches this stage, natural selection cannot operate on it. It is therefore absurd to look upon natural selection as the direct cause of the origin of the likeness. When once a certain degree of resemblance has risen, it is quite likely that in some cases natural selection has strengthened the likeness.

The second great objection to the Neo-Darwinian explanation of the phenomenon known as mimicry is that in many cases the resemblance is unnecessarily exact. Even as we saw how the Kallimas, or dead-leaf butterflies, carried their resemblance to dead leaves to such an extent as to make it appear probable that factors other than natural selection have had a share in its production, so do we see in certain cases of mimetic resemblance an unnecessarily faithful likeness.

The Brain-fever Bird

The common Hawk Cuckoo of India (Hierococcyx varius) furnishes an example of this: “The brain-fever bird,” writes Finn, on page 58 of Ornithological and Other Oddities, “is the most wonderful feather copy of the Indian Sparrow-hawk or Shikra (Astur badius). All the markings in the hawk are reproduced in the cuckoo, which is also of about the same size, and of similar proportions in the matter of tail and wing; and both hawk and cuckoo having a first plumage quite different from the one they assume when adult, the resemblance extends to that too. Moreover, their flight is so much the same that unless one is near enough to see the beak, or can watch the bird settle and note the difference between the horizontal pose of the cuckoo and the erect bearing of the hawk, it is impossible to tell them apart on a casual view.” Moreover, the tail of the cuckoo sometimes hangs down vertically, thus intensifying the likeness to the hawk.

It is quite possible that the brain-fever bird derives some benefit from the resemblance; indeed, it has been seen to alarm small birds, even as the hawk-like common cuckoo frightens its dupes, but, as D. Dewar pointed out, on page 105 of vol. 57 of the Journal of the Society of Arts, “this is not sufficient to explain a likeness which is so faithful as to extend to the marking of each individual feather. When a babbler espies a hawk-like bird, it does not wait to inspect each feather before fleeing in terror; hence all that is necessary to the cuckoo is that it should bear a general resemblance to the shikra. The fact that the likeness extends to minute details in feather marking, points to the fact that in each case identical causes have operated to produce this type of plumage.” This conclusion is still further strengthened by the fact that the likeness extends to the immature plumage, that is to say, exists at a time when it cannot assist the cuckoo in its parasitical work.

Poulton meets this objection as follows:

SHIKRA HAWK

The upper surface of the tail, not shown in this drawing, exactly corresponds with that of the cuckoo “mimic.”

HAWK-CUCKOO

This species (Hierococcyx varius) is commonly known in India as the “Brain-fever bird.”

Hypertely

“All such criticism is founded on our imperfect knowledge of the struggle for existence. The impressions and judgments of man are immensely influenced by the ‘corroborative detail,’ giving ‘artistic verisimilitude to a bold and unconvincing narrative.’ Indeed, the laughter which is invariably raised by this passage from The Mikado is, I have always thought, not only or chiefly due to the humour of the application, but to the way in which a great and familiar truth breaks in upon the listener with all the pleasing surprise which belongs to epigram. Birds, the chief enemies of insects, are known to have powers of sight far superior to those of man, and, from our experience of them in captivity, it may be safely asserted that their attention is attracted by excessively minute detail. Until our knowledge of the struggle for life is far more extensive than at present, the argument founded on Hypertely may be left to contend with another argument often employed against the explanation of cryptic and mimetic resemblance by natural selection. Hypertely assumes that there are unnecessary details in the resemblance, that the resemblance is perfect beyond the requirements of the insect; the second argument maintains that birds are so supremely sharp-sighted that no resemblance, however perfect, is of any avail against them. In the meantime the majority of naturalists will probably reject both extremes, and believe that the enemies are certainly sharp-sighted and successful in pursuit, but that perfection in detail makes their task a harder one, and gives to the individuals possessing it in a higher degree than others, increased chances of escape, and of becoming the parents of future generations.” (Essays on Evolution, p. 302.)

This long quotation requires careful consideration, since to us it appears to be typical of the kind of reasoning resorted to by Neo-Darwinians.

Note the reference to our “imperfect knowledge of the struggle for existence.” This is almost invariably the last refuge of the Neo-Darwinian when worsted in argument. We fully admit that there is still much to be learned of the nature of the struggle for existence, but such a statement sounds very curious when uttered to those who pin their faith to the theory which sees in the principle of natural selection an explanation of all the phenomena of the organic world. Natural selection, be it remembered, is but a name for the struggle for existence.

Birds capturing Butterflies

“Birds,” says Professor Poulton, “are the chief enemies of insects.” This may be so. But we greatly doubt whether they are the chief enemies of butterflies and moths, among which the most perfect examples of mimicry are supposed to occur.

We have watched birds closely for some years, but believe that we could almost count on our fingers the cases in which we have seen a bird chase a butterfly.

Professor Poulton, being aware of this objection, sets forth, on pp. 283-292 of Essays on Evolution, the evidence he has gathered in favour of the view that birds are the chief enemies of butterflies and other lepidoptera.

As the result of five years’ observation in S. Africa, Mr G. A. K. Marshall was able to record some eight cases of birds capturing butterflies. In three cases the butterfly seized was warningly coloured, or, at any rate, conspicuous! In two of these eight cases the bird failed to capture its quarry!

Says Mr Marshall, “the fact that birds refrain from pursuing butterflies may be due rather to the difficulty in catching them than to any widespread distastefulness on the part of these insects.”

During six years’ observation in India and Ceylon, Colonel Yerbury records some half dozen cases of birds capturing, or attempting to capture, insects. He writes: “In my opinion an all-sufficient reason for the rarity of the occurrence exists in the fact that in butterflies the edible matter is a minimum, while the inedible wings, etc., are a maximum.”

Colonel C. T. Bingham in Burma states that between 1878 and 1891 he on two occasions witnessed the systematic hawking of butterflies by birds, although he observed on other occasions some isolated cases.

This appears to be the sum total of the evidence adduced by Professor Poulton as regards the capture of butterflies by birds. This seems to us an altogether insufficient foundation upon which to build the theory that the cases of resemblance between unrelated species have been effected by natural selection.

It is, however, to be noted that probably among birds the most dangerous enemies of butterflies are not those that habitually catch insect prey on the wing. Such are experts in the art of fly-catching, and would despise the comparatively meatless butterfly. One often comes across butterflies with an identical notch in each wing, which leaves little room for doubt that those particular butterflies had been snapped at, while resting, by a bird. Among birds the chief enemies of butterflies and moths are probably to be found in those that hunt for their food in bushes and trees.

Thus, what we do know of the nature of the struggle for existence offers but poor support to the Neo-Darwinian explanations of the cases of so-called mimicry in nature.

Observing-powers of Birds

Professor Poulton’s idea of pitting the argument of Hypertely against that of the alleged supreme sharp-sightedness of birds is ingenious, but is not likely to satisfy very many people save those content to live in a fools’ paradise. If birds are supremely sharp-sighted, and pay attention to excessively minute detail, the difficulty of accounting for the origin of protective mimicry on the natural selection hypothesis becomes all the greater.

The question whether or not birds are good observers is a most interesting one. Unfortunately, hitherto, but little attention has been paid to the subject. The evidence available seems to point to the fact that birds, like savages, have sharp eyes only for certain objects—that is to say, for the things they are accustomed to look out for. All observers of nature must have noticed how quick a butcher-bird is to catch sight of a tiny insect upon the ground at a distance of some yards from his perch.

On the other hand, it is said that when there is snow upon the ground wood pigeons will approach quite close to a man wearing white clothes and a white hat, provided he keep perfectly still. Finn once witnessed in Calcutta a sparrow pick up a very young toad, obviously by mistake, for it dropped it at once with evident distaste. Birds of prey are supposed to have remarkably good eyesight; yet they can readily be caught by a net stretched out before their quarry. They are not trained to be on the watch for such things as nets, and so do not appear to notice one when erected.

It is thus our belief that the very perfection and detail of some so-called mimetic resemblances are a very serious objection to the theory of protective mimicry as enunciated by Professor Poulton and other Neo-Darwinians.

There is yet a further objection to this theory, one which, in our opinion, is fatal to the hypothesis in its generally accepted form.

A number of cases occur where two species, in no way related, show close resemblance to one another under such circumstances that neither can possibly derive any benefit from the likeness. The theory of protective mimicry is quite unable to explain these cases. This fact leads to a suspicion that, in the instances where the theory does at first sight appear to offer an explanation, the resemblance may also be due to mere coincidence.

We may perhaps call the cases which the theory of mimicry is unable to account for “false mimicry,” but in so doing we must bear in mind the possibility that some, at any rate, of the examples of so-called mimicry may, on further investigation, prove to be nothing of the kind.

“False” Mimicry among Mammals

The Cacomistle of Mexico (Bassaris astuta), one of the raccoon family, has a grey body and long black-and-white ringed tail, just like the ring-tailed Lemur of Madagascar (Lemur catta); both are arboreal and about the same size, and this lemur’s colouration is exceptional in its family.

The banded Duiker-buck of West Africa (Cephalophus doriae), has the same very unusual colouration as the thylacine or marsupial wolf of Tasmania, light brown, with bold black bands across the hinder part of the back, and the animals are about the same size.

The dormouse of Europe closely resembles a small American Opossum (Didelphys murina), and a larger opossum (D. crassicaudata) is very like the Siberian Mink (Mustela sibirica).

The Flying Squirrel of North America (Sciuropterus volucella) is closely copied by the Flying Phalanger (Petaurus breviceps) of Australia.

It will be readily seen that in no one of these cases can the likeness be of utility to either the “model” or the “copy.”

False Batesian Mimicry among Birds

There are many instances of this phenomenon among birds. The New Zealand Cuckoo (Urodynamis tritensis) shows a far closer resemblance to the American Sparrow-hawk (Accipiter cooperi) than to any New Zealand hawk, and in fact closely mimics this quite alien bird.

The stormy petrel, a purely oceanic bird, closely resembles in size, colour, and style of flight the Indian Swift (Cypselus affinis), a purely inland creature; both are sooty black, with a conspicuous white patch on the lower back.

The Pied Babbling Thrush (Crateropus bicolor) of Africa is singularly like the Pied Myna (GrÆulipica melanoptera) of Java, both being of about the same size, with white body and black wings and tail quills. This, we may add, is a very unusual colouration among small birds.

The black-headed Oriole (Oriolus melanocephalus) of India is very similar in appearance to the common Troupial (Icterus vulgaris) of Brazil; indeed, the troupials, a purely American group, are so like the old world orioles in colour that they usurp their name in America.

The little insectivorous Iora (Ægithina tiphia) of India strongly resembles in size and colour a Siskin (Chrysomitris colambiana) from South America, the males in both being black above and yellow below, while in the females the black is replaced by olive-green.

Another Indian babbler (Cephalopyrus flammiceps), yellowish-green, with orange forehead, is closely copied by, or copies, the well-known Brazilian Saffron-finch (Sycalis flaveola).

In Fergusson Island, near New Guinea, there is a ground pigeon (Otidiphaps insularis) which is black with chestnut wings, like several of the powerful ground cuckoos of the genus Centropus, but no species of these cuckoos so coloured appears to inhabit the island.

In Africa there is a tit (Parus leucopterus) which has the same very unusual colouration as an East-Indian bulbul (Micropus melanoleucus), both being black with a white patch on the wing-coverts. These two birds are about the same size. As showing the purely coincidental character of such resemblances, we may mention that this same rare pattern occurs again in our Black Guillemot (Uria grylle) and in the Muscovy Duck (Cairina moschata).

We have already quoted Gadow (p. 198) on “false mimicry” among snakes. He also gives, on p. 110 of Through Southern Mexico, an example of this phenomenon among amphibia. It is, he writes, “impossible to distinguish certain green tree-frogs of the African genus Rappia from a Hyla, unless we cut them open. If they lived side by side, which they do not, this close resemblance would be extolled as an example of mimicry.”

We should be very greatly surprised if abundant examples of “false mimicry” are not found among insects. We trust that this remark will stimulate some entomologist to pay attention to the subject.

It is the essence of MÜllerian mimicry that both model and copy are immune from attack from enemies. Unfortunately for the theory, similar resemblances occur among birds of prey, where neither party can benefit from the association. This gives rise to what we may perhaps call false MÜllerian mimicry. Thus the goshawk and peregrine falcon resemble each other in being brown above and streaked below in immature plumage, and having barred underparts and a grey upper plumage when adult.

Theory of Mimicry Criticised

Having stated the more important objections to the theory of protective mimicry, it now remains for us to deal specifically with each head of evidence offered in its favour.

1. With regard to the assertion that the model and its copy are often not nearly related, we have shown that among mammals and birds instances of resemblance between widely-separated groups occur under such circumstances that neither party can derive any benefit therefrom.

2. As regards the assertion that species which are mimicked are either well-defended or unpalatable, this certainly does not hold good with regard to some at any rate of the coincidental resemblances among birds which we have pointed out; even if these pairs of similar species lived in the same country it would require considerable ingenuity to say why one should mimic the other.

3. As regards the argument that the inedible species of IthomiinÆ, etc., display only fifteen colours, while the less numerous edible Papilios display more than double this number of colours, we may draw attention to the fact that those birds which are most immune from attack are precisely those which display the smallest range as regards colour, e.g., hawks, owls, crows, gulls, storks, and cranes. As we have already submitted, no question of MÜllerian association comes in here.

On the other hand, the eminently edible families of game-birds and ducks display great variety of colour, in the males at all events.

4. As regards the statement that although in many cases the mimetic resemblances extend to the minutest detail, they are not accompanied by any structural changes except such as assist in the production of a superficial likeness, we may refer to the case we have already cited of the New Zealand cuckoo, which, though it so closely copies an American hawk, is typically cuculine in structure. Here, of course, there can be no question of advantage to the “mimicking” cuckoo in the resemblances.

5. In answer to the argument that mimetic resemblance extends to form, attitude, and movement, as well as colour, and that deep-seated organs are affected only when the superficial resemblance is thereby intensified, we may draw attention to such cases as the following:—

(a) The harmless Indian Snake (Lycodon aulicus) is closely similar to the well-known Krait (Bungarus coeruleus), also Indian; but the resemblance extends to a structural detail which can hardly have mimetic value—namely, the harmless snake has long, fang-like front teeth, though these are unconnected with poison-glands. Animals which come into contact with the krait and its mimic are hardly likely to inspect their teeth.

(b) A considerable number of birds of the shrike group—known as Cuckoo-Shrikes (Campophaga)—closely resemble cuckoos in plumage; but even if they derive any benefit from mimicking birds which are credited with being mimics already, they cannot profit by the fact that the shafts of the rump-feathers in both groups are stiffened; this being a peculiarity which would not be perceptible until the bird was in the grasp of an aggressor.

(c) As a third case of coincidence we may refer to the tubercle in the nostril of the Brain-fever-bird (Hierococcyx varius), as a minute detail of hawk-like appearance, though not present in the particular species imitated.

6. The argument that mimetic resemblances are produced in the most diverse ways, but the result is uniform, loses much of its force when we consider the various methods by which short-tailed birds appear to have long caudal appendages.

In the peacock it is the upper tail coverts which are elongated; in the Stanley Crane (Tetrapteryx paradisea) it is the innermost or tertiary quills of wing; in one of the egrets some of the feathers of the upper back grow to a great length and form a train; in the Bird of Paradise (Paradisea apoda) the long flank plumes are commonly mistaken for the tail.

In these cases there can be no question of mimicry.

7. We have shown that the idea that imitator and imitated are always found in the same area is absolutely fallacious. In birds, for example, the most striking resemblances appear to occur between species that dwell far apart.

8. We can cite, as parallel to the case of a mimicking species of which the male copies one model and the female another, the strange similarity between the barred brown plumage of the female blackcock and that of the female eider-duck. The males of these species, although both black and white, differ greatly in appearance; but the male blackcock is admittedly very like the male of another species of sea-duck—the scoter.

9. Against the supposed ancestral non-mimetic forms existing on islands we can pit the “mimetic” orioles in small islands and their non-mimetic cousins on the mainland. In Australia an oriole of what appears to be an ancestral style lives beside, but declines to mimic, a friar bird of a very pronounced type.

10. The case of certain diurnal moths mimicking butterflies appears to be explicable without the aid of the theory of protective mimicry. When two species adopt the same method of obtaining food, it not infrequently happens that a professional likeness springs up between them. Of this the swifts and swallows afford a striking illustration.

11. As a set-off to the cases where the alleged mimicry is confined to certain seasons of the year, we may cite the case of the pheasant-tailed JaÇana (Hydrophasianus chirurgus), which in its winter plumage might easily be mistaken, when on the wing, for the paddy bird or Pond Heron (Ardeola grayii), both being of like size and having a brown back, long green legs, and white wings. Moreover, they are to be found in the same localities in India. At the breeding season, however, they are absolutely different in plumage.

Yet another argument commonly adduced in favour of the theory of protective mimicry is that local variations of the imitated species are sometimes followed by the imitator; thus the butterfly Danais chrysippus shows a white patch on the hind wings in Africa, and this is followed by its mimic.

But the same thing occurs, quite irrationally, so to speak, among birds. The peregrine falcon and hobby of Europe are only winter migrants to India, where they are replaced as residents by the Shaheen (Falco peregrinator) and Indian Hobby (F. severus). Both these differ from the migratory forms by being blacker above and chestnut below, instead of cream colour. Thus the resemblance occurs in each race. A similar distinction, as noted by Blyth, exists between the Common Swallow (Hirundo rustica) and the Swallow (H. tytleri) of Eastern Asia, the latter having the whole ventral surface rufous instead of only the throat. Yet no one will suggest that swallows mimic falcons, or that there is mimicry between the peregrine and hobby. It is obvious that such parallel changes occur independently of mimicry.

The Water-rail (Rallus aquaticus) and Baillon’s Crake (Porzana bailloni) of Europe are distinguished from their allies of Eastern Asia by having the sides of the head plain grey, whereas the Eastern Asiatic forms (R. indicus and P. pusilla) have a brown streak along each side of the face. Here, again, we have an instance of birds of the same family varying together with geographical distribution.

“Recognition” Colours

One of the prettiest conceits of the Wallaceian school of zoologists is the theory of recognition markings.

“If,” writes Wallace, on page 217 of Darwinism, “we consider the habits and life-histories of those animals which are more or less gregarious, comprising a large proportion of the herbivora, some carnivora, and a considerable number of all orders of birds, we shall see that a means of ready recognition of its own kind, at a distance or during rapid motion, in the dusk of twilight or in partial cover, must be of the greatest advantage and often lead to the preservation of life. Animals of this kind will not usually receive a stranger in their midst. While they keep together they are generally safe from attack, but a solitary straggler becomes an easy prey to the enemy; it is therefore of the highest importance that, in such a case, the wanderer should have every facility for discovering its companions with certainty at any distance within the range of vision.

“Some means of easy recognition must be of vital importance to the young and inexperienced of each flock, and it also enables the sexes to recognise their kind and thus avoid the evils of infertile crosses; and I am inclined to believe that its necessity has had a more widespread influence in determining the diversities of animal colouration than any other cause whatever. To it may probably be imputed the singular fact that whereas bilateral symmetry of colouration is very frequently lost among domesticated animals, it almost universally prevails in a state of nature; for if the two sides of an animal were unlike, and the diversity of colouration among domestic animals occurred in a wild state, easy recognition would be impossible among numerous closely allied forms.”

As examples of recognition colouration, Wallace cites, among others, the white upturned tail of the rabbit—a “signal flag of danger,” the conspicuous white patch displayed by many antelopes, the white marks on the wing- and tail-feathers of the British species of butcher-birds, the stone-chat, the whin-chat, and the wheat-ear.

Wallace therefore asserts, firstly, that recognition marks not only help herbivorous animals to keep together, but act as a danger signal; the member of a flock which first catches sight of the enemy takes to its heels, displaying its white flag, which is the signal of danger to the other members of the flock. Secondly, that recognition marks prevent the evils of infertile crosses. Thirdly, that the necessity of being able to recognise one another has rigidly preserved bilateral symmetry among animals in a state of nature.

As regards assertion number one, we would point out that where a flock of herbivora is being stalked by a beast of prey, the member of the flock nearest to the enemy—that is to say, the hindmost member—will probably be the first to observe him. As that creature will be more unfavourably situated for escape than the rest of the herd, it will not be to their advantage to follow the line it has taken. Moreover, being at the rear of the flock, it is not in a good position to take the lead, and its pursuer is likely to see the danger signal before its friends do. It would thus seem that “danger signals,” while possibly sometimes of service to their possessors, are on the whole ornaments which might profitably be dispensed with. Natural selection can scarcely be charged with the production of a character of such doubtful utility to the organism.

Moreover, flourishing species of many gregarious animals do not possess any “signal flag of danger,” while, on the other hand, a great many solitary species display markings that render them very conspicuous when in motion. Take the case of the famous Indian Paddy Bird (Ardeola grayii). This, when at rest, is coloured so as to be very difficult to distinguish from its surroundings, but flight transforms it, for it then displays its milk-white pinions, which would make a perfect danger signal, if only it were not peculiarly solitary in its habits. Its gregarious brethren, the Cattle Egrets (Bubulcus coromandus), on the other hand, display no danger signal.

Interbreeding of Allied Species

That these recognition marks prevent the intercrossing of allied species and the production of infertile hybrids appears to be pure fiction. As we have already shown, hybrids between allied species are by no means always infertile. Moreover, species which differ only in colour seem usually to interbreed in those parts where they meet.

“This interbreeding,” writes Finn, on page 14 of Ornithological and Other Oddities, “occurs where the carrion crow (Corvus corone) meets the hooded crow (Corvus cornix), where the European and Himalayan goldfinches (Carduelis carduelis and C. caniceps) encounter each other, and where the blue rollers of India and Burma (Coracias indicus and C. affinis) come into contact, to say nothing of other cases.”

Of these other cases, the Indian bulbuls of the genus Molpastes form a very remarkable one. In all places where two of the so-called species meet they appear to interbreed, and so freely do they interbreed that at the points where the allied species run into one another it is not possible to refer the bulbuls to either species. Thus William Jesse writes of the Madras Red-vented Bulbul (Molpastes hÆmorrhous) (page 487 of The Ibis for July 1902): “This bird, although I have given it the above designation, is not the true M. hÆmorrhous. I have examined numbers of skins and taken nests and eggs time after time, and have come to the conclusion that our type is very constant, and at the same time differs from all the red-vented bulbuls hitherto described. The dimensions tally with those given by Oates for M. hÆmorrhous, while the black of the crown terminates rather abruptly on the hind neck, and is not extended along the back, as is the case with M. intermedius and M. bengalensis. On the other hand, as in the two last species, the ear coverts are chocolate. Furthermore, I may add—although I attach little importance to this—that the eggs of the Lucknow bird which I have seen are, without exception, far smaller than my eggs of genuine M. intermedius from the Punjab. My own opinion is that the Lucknow race is the result of a hybridisation between the other three species.”

Further, in Bannu, Mr D. Donald saw M. intermedius and M. leucogenys paired at the same nest. That gentleman could not possibly be mistaken on the point, as the latter species has white cheeks and yellow under tail-coverts, while the cheeks of the former species are dark-coloured and the patch of feathers under the tail is red. Similarly, Whitehead and Magrath, writing of the birds of the Kurram Valley (Ibis, January 1909), record that the former shot no fewer than twelve bulbuls, which undoubtedly appear to be hybrids between these two species. As these hybrids differ considerably inter se, there seems no room for doubt that they breed with one another and with the parent species.

Symmetry in Nature

Wallace’s third statement, that if the two sides of animals in a state of nature were alike, easy recognition would be impossible among numerous closely allied forms, reminds us forcibly of the sad case of the boy whose tailor was his mother. Humanum est errare: she made her son one pair of trousers that fastened up behind, so that the poor boy when wearing them never knew whether he was going to or coming home from school! If animals are able to recognise their mates, their bilateral symmetry does not seem necessary to enable them to distinguish their fellows from allied species.

It is, indeed, true that asymmetrically marked animals are very rarely seen in the wild state, while they are the rule rather than the exception among domesticated species. But this appears to be due, not to the necessity of recognition markings in nature, but to the fact that those animals that display a tendency to massed pigment perish in the struggle for existence, since this massing of pigment appears to be correlated with weakness of constitution. In other words, this massing of pigment is an unfavourable variation, which under natural conditions dooms its possessor. In the easier circumstances of domestication, animals which are irregularly pigmented are able to survive, so that, among them, the almost universal tendency to the massing of pigment can be followed without let or hindrance.

It is unnecessary to say more upon this subject. The few facts we have set forth suffice to destroy this particular excrescence on the Darwinian theory.

The Colouring of Flowers and Fruits

Extremely interesting though the subject be, we are unable to consider at length the generally accepted theory that the colour markings and perfumes of wild flowers are the result of the unconscious selection exercised by insects.

While not denying that many flowers profit by their colouring, that these colours may sometimes serve to attract the insects, by means of which cross-fertilisation is effected, we are not prepared to go to the length of admitting that all the colours, etc., displayed by flowers and floral structures are due to the unconscious selection exercised by insects. It is one thing to admit that the colour of its flowers is of direct utility to a plant; it is quite another to assert that the colour in question owes its origin and development to natural selection. Our attitude towards the generally accepted explanation of the colours of flowers is similar to that which we adopt towards the theory of protective mimicry among animals. In certain cases we are prepared to admit that the mimicking organism derives benefit from the likeness; but this, we assert, is no proof that natural selection has originated the likeness.

Cross- versus Self-fertilisation

The theory that flowers have developed their colours in order to attract insects to them, and thus secure cross-fertilisation, is based on the assumption that cross-fertilisation is advantageous to plants. It is questionable whether this assumption is justified. True it is that numbers of experiments have been performed, which show that, in many cases, flowers which are artificially self-fertilised yield comparatively few seeds. But experiments of this kind do not prove very much.

To place on the stigma pollen from the anthers of the same flower, in case of a plant which for many generations has been cross-fertilised, is to subject the plant in question to a novel experience—an experience which may be compared to transplanting it to another soil. The immediate effect may appear to be unfavourable, although, if the experiment be persisted in, the ultimate results may prove beneficial to the plant.

That this is the case with some flowers that are artificially fertilised is asserted by the Rev. G. Henslow. This observer states, that had Darwin pursued his investigations further, he would probably have modified his views regarding the benefits of self-fertilisation. Darwin’s statement that “Nature abhors perpetual self-fertilisation” seems to be as far from the truth as that which declares “Nature abhors a vacuum.”

From the mere fact that cross-fertilised flowers yield a greater quantity of seed than they do when self-fertilised, it does not necessarily follow that cross-fertilisation is advantageous. The amount of seed produced is probably not always a criterion as to the advantages of the crossing to the plant. Some flowers yield most seed when fertilised by the pollen from flowers belonging to a different species!

It is significant that some plants produce cleistogamous flowers, that is to say, flowers which invariably fertilise themselves. Such flowers never open; so that the visits of insects are precluded.

According to Bentham, the Pansy (Viola tricolor) is the only British species of Viola in which the showy flowers produce seeds. The other species are all propagated by their cleistogamous flowers. The genus Viola is an advanced species: it would therefore seem that the production of cleistogamous flowers is an advance on the production of entomophilous flowers. Cleistogamous blossoms are obviously more economical.

Insects and Flowers

In the case of the malvas, epilobias and geraniums, where we see, side by side, races of which the individuals produce insect-fertilised flowers and those that are characterised by self-fertilised flowers, the latter are quite as thriving as the former.

The common groundsel, which, according to Lord Avebury, is “rarely visited by insects,” flourishes like the green bay tree, as many gardeners know to their cost. The same may be said of the pimpernels. In this connection it is important to bear in mind that the anemophilous, or wind-fertilised, angiosperms, as, for example, the grasses, are believed to be descendants of insect-fertilised or entomophilous forms.

A weighty objection to the theory that the colours of flowers have been developed because they attract insects has been urged by Mr E. Kay Robinson, namely, that among wild flowers the most highly coloured ones are the least attractive to insects.

“Show me,” writes he, on page 222 of The Country-Side for March 20, 1909, “the insect-collector who will seek for specimens among the brilliant scarlet poppies. Of what use is the dog rose, with its large discs of pinky-white, to him? On the other hand, does he not find that by far the most attractive flowers are the almost invisible spurge laurel blossoms in February and March, the fuzzy sallow catkins in March and April, the bramble blossom in midsummer, and the ivy’s small green flowers in autumn? Of these only the bramble has any pretensions to colour, and if you try, as I have tried, the experiment of picking off every petal from sprays of bramble blossoms you will find that its attraction to moths does not appear diminished.

“The fact that insects do visit many conspicuously coloured flowers does not show that the colour attracts them, when the fact is borne in mind that they neglect others which are equally coloured, while the flowers which they particularly haunt are inconspicuous. Conspicuous flowers which have abundance of nectar attract insects, of course, but so do inconspicuous flowers which have nectar. If they have no nectar, neither the conspicuous nor the inconspicuous flowers attract insects other than pollen or petal eaters, whose visits are not good for the plant. This shows that the nectar attracts the insects and that the colour of the flowers makes no difference.”

In autumn many leaves assume bright and beautiful tints. These are not believed to be in any way useful to the plant. The autumnal hues and shades are regarded, and rightly regarded, as the garb of death and decay. Such colours are the result of the oxidation of the chlorophyll or green colouring matter of the leaves. Why should not the colours of the petals of the flowers, which wither and fade long before the green leaves do, be due to a similar cause? The bright colours of fruits are supposed to have been effected by natural selection in order to attract fruit-eating animals. Surely a hungry animal does not require that its food be brightly coloured in order to find it! We must remember that during the greater part of the year most animals have no occupation save that of finding their food. Inconspicuously coloured fruits, like those of the ivy, are frequently eaten by birds. The bright colours of some ripening fruits are undoubtedly the colours of decay. Many fungi and seaweeds have bright colours. It is never hinted that these are of any direct utility to their possessor.

Every flower, every plant, every organism must be of some colour.

Honey

Many flowering plants produce honey. This is said by some botanists to have been directly caused by natural selection, because the honey attracts insects. Possibly those who take up this attitude are putting the cart before the horse. It is probable that honey, like oxygen, is an ordinary product of the metabolism of the plant, and that the visits of bees and other insects to such plants are the result rather than the cause of the honey being there. Boisier found that some plants, for example, Potentilla tormentilla and Geum urbanum, gave honey in Norway, but very little near Paris.

He further discovered that by supplying certain plants copiously with water he could induce them to produce more than their normal output of honey.

As is their habit, Neo-Darwinians have pushed their pet theory to absurd lengths in its application to flowers. They assert that the visits of insects are responsible for not merely the general colour of every flower, but also the various lines, spots, and other markings of flowers. The lines that frequently occur on the petals are supposed to guide the insects to the honey! This particular refinement of Neo-Darwinism, to quote Kay Robinson, “needs little discussion. Insects have very poor sight. You can see this when a bee or a butterfly flies bang against a whitewashed wall; when a wasp pounces upon a black spot on a sunlit floor, mistaking it for a fly; or when a settled dragon-fly will allow you to poke it in the face with the end of a walking-stick, although it will be off like a flash if you raise your arm. There is, therefore, large reason to doubt whether insects can even see the fine lines in the throats of flowers which are supposed to guide them to the nectar. It is rather absurd, too, to suppose that such lines can be needed, since insects come in swarms to inconspicuous and apparently scentless flowers or to ‘sugared’ tree-trunks in the dark. Where there is nectar, insects which have come to the feast from a distance need no pencilled lines to guide them over the last quarter of an inch of their journey.”

Scents of Flowers

Neo-Darwinians further assert that the scents of flowers have been developed by natural selection because they serve to attract insect visitors to the flowers. In support of this contention it is urged that the most highly scented flowers are not usually the most conspicuous ones, since it is not necessary for a flower to be both highly coloured and strongly scented. Again, those flowers which open at night are usually very highly scented.

Plausible though this view seems, there are weighty objections to it. These are so admirably summarised by Kay Robinson in the issue of The Country-Side for March 27, 1909, that we feel we cannot do better than reproduce his words:—

“It is true that many flowers which are strongly scented are visited by insects, but these flowers have abundance of nectar, and the insects come in spite of the scent, and not on account of it. They visit unscented flowers, provided that they have nectar, equally freely; and they do not visit flowers which have scent without nectar.

“Moreover, fruits are more generally scented even than flowers; but what explanation have those, who attribute the scents of flowers to the tastes of insects, for the scents of fruits? Insects which visit fruits are only robbers. Therefore, if we say that plants have scents for the purpose of attracting insects, we accuse all plants which have scented fruits of attempted suicide.

“There are hosts of plants, again, with scented leaves. Here also the insects are only robbers, and it is quite clear that the scent is not useful in attracting insects. If, therefore, you adopt the insect theory to explain the scents of flowers, you must invent entirely new theories to explain the scents of fruits and leaves.”

It is thus evident that the ordinarily accepted explanation of the colours, scents, and markings of flowers is far from satisfactory.

Kay Robinson’s Theory

Mr E. Kay Robinson has put forth in recent issues of The Country-Side (March 20, 27, and April 3, 1909) quite a new explanation of the phenomena, and one which deserves careful consideration. He maintains that “the real, primary, and original meaning of the colours, markings, nectar and scents of flowers is not to attract insects, but to deter grazing and browsing animals.”

“I say,” he writes, “that grazing and browsing animals avoid eating conspicuous flowers. I have watched a flock of five hundred sheep pass across a yard-wide strip of close-nibbled turf on the Norfolk coast, grazing as they passed, and the number of open daisy blossoms after they had passed seemed the same as before they came. Every one of five hundred sheep had eaten something from that yard of grass, and not one had eaten any of the hundred and thirty odd daisies.

“Every summer the farm horses are turned into the same old pasture, and as the summer wanes the field always presents the same appearance—the green grass close-grazed, the tall buttercups left standing high.

“Once, leaning over a gate with friends, I pointed out that a flock of sheep grazing in a sainfoin field were nibbling the greenstuff close, but were not eating the flowery stalks, when one sheep near us accidentally pulled up a whole sainfoin plant by the roots and proceeded to munch it upwards. Inch by inch the stem passed into its jaws, and I began to be afraid that it was going to establish an ‘exception’ to my rule. But, just when the bright cluster of pink sainfoin blossom was within two inches of its teeth, it gave an extra nip, and the flower head fell to the ground, and the sheep resumed its search for greenstuff.

“I do not say that this would always happen—I should be sorry for any theory which depended upon the intelligence of a sheep—but it was a very striking object-lesson to my two companions; and any one who looks around during this summer with an inquiring mind will find plenty of evidence that grazing, browsing, and nibbling animals avoid flowers, and stick to greenstuff when they can get it.

“I do not say that all animals avoid the same flowers. Horses, for instance, may dislike large flowers like roses and conspicuous yellow flowers like buttercups, but they will bite off flat clusters of minute white or pale yellow flowers, such as yarrow or wild parsnip. These distinctions made by certain kinds of beasts will probably in the future be found to afford valuable evidence as to the regions of origin of our flowers and animals. Such plants as the yarrow and the wild parsnip, for instance, probably did not originate in the home of the wild horse, because they are not protected against it.

“As a general rule, however, there is abundance of evidence that plants with conspicuous flowers gain a large advantage in the struggle for existence, because grazing and browsing animals avoid them; while there is no real evidence at all that conspicuous flowers attract insects.”

Kay Robinson extends this explanation to the shape, the scent, and the nectar of flowers. He admits that many flowers are adapted to the visits of insects, but this is, he asserts, but a secondary result. The “real, primary meaning” of the shapes of flowers of curious configuration is, he insists, “a deterrent to grazing or browsing animals.”

According to him plants, like the snap-dragon, which have “blossoms in the semblance of a mouth,” are avoided by grazing animals, because they mistake such flowers for mouths, and have no wish to be bitten! Orchids, he asserts, “are strongly deterrent to grazing and browsing animals, which are looking for greenstuff, and regard these gaudy, spidery, winged blossoms as live creatures.” “If this is not the truth,” he asks, “will any adherent of the theory that we owe the shapes of flowers to insects explain why some of our common British orchids are so like bees, spiders, etc.? Some which have no particular resemblance to any insect still exhibit weird shapes, suggestive to the human mind of living things, such as lizards, etc. The reason why they look like bees, spiders, lizards, and various unclassed creatures is quite simple. Grazing animals are looking for greenstuff, and do not wish to eat living creatures which may bite or sting or taste nasty. Thus the orchids have acquired the power of looking like creatures.

“Every one,” he continues, “who is familiar with the blossom of the wild carrot—a flat head of minute, dull-white blossoms—must have noticed how very often the centre blossom in each head is purplish or reddish-black. This makes it very conspicuous in the middle of the flat white flower head. Now what conceivable use can this barren little blackish blossom—scarcely bigger than a pin’s head—be to the wild carrot plant if we regard the flat head of white flowers as an attraction to the sight of insects? If, on the other hand, we rightly regard the flat head of white blossoms as an advertisement to grazing animals that it is not wholesome greenstuff, but innutritious blossoms liable to be infested with ants and other stinging insects, we see at once the great use of this small blackish flower in the middle. It looks like an insect, and possibly in the home of the wild carrot there is some minute blackish insect with a peculiarly villainous smell or taste—or perhaps a potent sting—which grazing animals carefully avoid whenever they can see it. Thus the wild carrot flourishes; though here in Britain—where the wild carrot has established itself now—we may fail at first to see the exact meaning of the trick. I think, however, that, when we understand it, it fits admirably into the theory that the shapes and colours of flowers are primarily useful as deterrents to grazing and browsing animals and not as attractions to insects.

“Thus we see,” he concludes, “that the queer shapes of these orchids, which are a great stumbling-block in the way of those who preach that we owe the shapes of flowers to the tastes of insects, become a strong confirmation of my theory that we owe the shapes of flowers to grazing and browsing animals.”

Of the nectar of flowers, Kay Robinson writes: “Since this is eagerly sought for by hosts of insects, whose visits are in most cases useful to the flowers, it seems only natural to suppose that we see cause and effect in this connection.

“Here, however, I will outline my theory of the origin of nectar and of flowers in general.

“I think there is no doubt whatever that all the parts of a flower are modified leaves. The original type of flowering plant—I think we may safely assume—had a single stem and produced its seed at the summit, as the crown of its year’s endeavour. The flower, before it became what we would recognise as a flower, was a cluster of protecting leaves round the seed-making parts of the plant. To the production of the seed the whole energies of the plant were devoted, and into the cluster of leaves at the top of the stem all the essences of the plant were concentrated. If during the coming spring you handle and examine the leaves at the end of the strong shoots of thorns or fruit bushes, you will find that the surface of the young leaves is quite sticky. If you observe browsing animals also, you will discover that—contrary to expectation—they do not like strong-growing, juicy shoots, evidently preferring mature leaves lower down the branch. This shows, I think, that plants have the power of protecting their new shoots by crowding into them the volatile oils and essences which they produce as a protection against animals. Now nectar appears always to be distasteful to grazing and browsing animals; and they also dislike scented flowers. I think, therefore, that it is reasonable to suppose that the nectar and scents which now distinguish so many flowers were first produced as an exudation of concentrated sap upon the surfaces of the protecting leaves round the seed-making parts of the original flowers. As these leaves became more efficiently protective by assuming colours, shapes, and markings which warned animals of their character, so their apparatus for producing scent and honey became specialised; and at this point the insect appeared upon the scene as a factor in the life’s success of the plant.”

Such, then, is Kay Robinson’s bold and original theory. In some respects it seems far-fetched. The natural inclination is to ask, “Is it possible that cattle can be so stupid, so blind, as to really believe that a snap-dragon is the mouth of an animal, or that an orchid is a spider?”

At present we know so little of animal psychology that we are not yet in a position to give an answer to this question. Horses, we know, are apt to be frightened by the most harmless things, such as a piece of brown paper lying on the road. Mr Robinson’s theory should give a stimulus to the study of the mind of animals—a study which, if properly undertaken, will probably throw a flood of light upon some of the problems of evolution. Mr Robinson’s theory equally with the ordinarily-accepted hypothesis, utterly fails to explain the first origins of colours, scents, etc. When once a flower has acquired a certain amount of colour, it is easy to understand how that flower may attract insects or repel grazing animals. But how can the origin of the colour or other characteristic be explained?

We asked Mr Kay Robinson how he would account for the great success in the struggle for existence of some species of grasses on which herbivorous animals feed so largely. He replied, in the issue of The Country-Side, dated April 3, 1909:—

“The grass has a manner of growth which defies the grazing animal. Its long, thin leaves are constantly pushing upwards from the ground, and, if they are grazed down one day, they will have pushed up again the next. Moreover, when the outside blade of grass has exhausted its power of growing, there is another blade inside it with many inches still to grow, and another inside that which has scarcely begun to grow, and yet another further in which has not yet seen daylight; and so on. In a state of nature grazing animals are nowhere so numerous on any given patch of ground from day to day as to keep down the grass. If they were, carnivorous animals would stay there to eat the grazing animals, and grow fat and multiply. Thus the grazing herds are scattered and wandering, followed wherever they go by the beasts of prey; and in their absence the grass pushes ahead, so that when the grazing animals return its clump is larger and its roots are stronger, and it is better able to survive attack than before.

“The method of the clovers and trefoils is quite different. When circumstances are favourable and enemies few, they will form large-leaved luxuriant clumps, with fine heads of blossom; but where grazing animals abound they have the power of adapting themselves to altered circumstances. They creep so closely along the ground that the teeth of the grazing animal cannot pick them up between the surrounding grass, and they produce leaves so small and short-stalked that to eat them would be like nibbling the pile off velvet. Any clover or trefoil thus growing in self-defence is accepted as the ‘shamrock’ of Ireland; and it is certainly a fine emblem for a race which regards itself as surviving in spite of incessant oppression.

“These are the reasons, however, why the grasses and clovers or trefoils continue to enrich old pastures when most of the other plants disappear, with the exception of daisies and buttercups, and the acid sorrels.”

We should be glad to hear how Mr Robinson accounts for the conspicuous flowers in the species of “prickly pear” (Euphorbia), which is so abundant in India, and which is not browsed upon by animals.

We regret that we are not able to devote more space to this most interesting theory. We can only add that, even if it fail to become widely accepted, it is of great value as showing that it is possible to offer a plausible explanation of a large number of phenomena, which nine out of ten botanists explain in a very different way.

So satisfied are the majority of naturalists with the “insect theory,” that they seem of late years to have paid but little attention to the subject of floral colouration. This affords a striking instance of the pernicious influence which Neo-Darwinism is exercising on the minds of men to-day. It tends to stifle research instead of stimulating it.

Accepted Theories Unsatisfactory

We have now dealt with the theory of protective colouration, the theory of warning colouration, the theory of mimicry, and the theory of recognition markings. We have shown that although many organisms undoubtedly derive profit from the fact that they are difficult to see in their natural surroundings or from their resemblance to other organisms, the hypothesis that this inconspicuousness or the mimicry of these animals has been caused by the natural selection of small variations is untenable.

Warning colours, we have shown, although a disadvantage to their possessors, are sometimes seen in nature because they are accompanied by unpalatability. The theory of recognition markings must, we fear, be laid to rest in the burial ground of exploded hypotheses.

The extreme popularity of the existing theories regarding animal colouration and their very general acceptance are to be attributed, firstly, to their simplicity; secondly, to the fact that they have thrown light on many phenomena which previously had seemed inexplicable; thirdly, that if we assume, as the great majority of biologists do, that evolution has been effected by the accumulation of numerous variations, small in degree and indefinite in direction, we seemed forced either to accept Neo-Darwinism or admit that the whole subject of animal colouration baffles us, in other words, to reject what appears like cosmos and substitute for it chaos.

With a few exceptions, books that deal with the colours of organisms, while emphasising the evidence in favour of the generally-accepted theories, seem almost entirely to ignore the host of facts that do not appear to fit in with them.

This is largely due to the almost unavoidable bias of the human mind when obsessed by a pet theory. There are none so blind as those who will not see. It is also, in part, the consequence of the prevalent neglect of the scientific method of comparison which leads men to theorise on insufficient evidence. This, of course, is a natural result of specialisation in biology. Naturalists are in the habit of confining their study to the habits of the animals of one particular country and then making far-reaching generalisations therefrom.

As an example of the kind of theorising to which this method leads, we may cite the often-quoted theory which ascribes the green colouring of some arboreal fruit-eating pigeons to adaptation to an existence among tropical foliage, and ignores the fact that in America tree-haunting pigeons are never of this colour, and that it is not by any means universal even among the old-world pigeons.

White Down of Nestlings

Similarly, a theory has been advanced (W. P. Pycraft, Knowledge, 1904, p. 275) that the white down of some nestling birds, is an adaptation to resisting the heat of the sun in open nests. This is at once negatived by the fact that young owls, usually hatched in shaded places, are also generally white, while young cormorants, living in open nests, are black; yet the allied darters, with the same breeding haunts in some cases, have white young. Lest it should be thought that black has some especial value in a nestling living exposed, we may mention that young petrels, which are born in holes, have black or dark down.

As we have already pointed out, naturalists in too readily accepting the theory that variation is minute in degree and indefinite in direction, have raised quite unnecessary difficulties, even for the selection hypothesis. We have cited certain facts, which seem to show that variations, as a rule, are not indefinite in direction; of these the most striking is furnished by birds in which the tail feathers are greatly elongated. Were variations indeterminate, we might reasonably expect to find that the elongation occurred in one particular feather or pair of feathers in one species, in another pair in a second species, in a third pair in a third species, and so on. But this is not the case; no bird has one single long feather in its tail, and when two are elongated, as is so commonly the case, these are almost invariably the middle or the outside pair; e.g., in the European bee-eater and pheasant it is the former, in the swallow and blackcock, the latter.

Exceptions are so rare that they may almost be said to prove the rule; e.g., although most terns have the outer-tail feathers elongated, in some of the Noddy Terns (Anous, Gygis) the third pair, in others the fourth pair, of tail feathers are the longest. This must mean one of two things, either that the variation, as regards length in tail feathers, other than middle or outer, does not ordinarily occur, or that it occurs, but is, in some way, inimical to the welfare of the species. The latter hypothesis does not seem probable, as the Noddies are particularly abundant birds where they occur, that is to say, in the tropical seas; therefore, we can only conclude that that particular variation has not occurred in birds as a whole.

We have adduced abundant evidence to show that mutations or discontinuous variations occur in nature; and as these afford much more favourable material on which natural selection can act, it is reasonable to suppose that they have played a considerable part in evolution.

When discussing the phenomena of inheritance, we attempted to show that, not improbably, these discontinuous variations are due to some re-arrangement in the constituent parts of the unit characters, or biological molecules, as we have called them.

Cranes

In this connection we may mention the apparently singular phenomenon of different species in the same natural group, exhibiting either a definite excess or deficiency of plumage on the head. Among cranes, most species are more or less bald; but the Demoiselle (Anthropoides virgo) has a fully-feathered head with long side-plumes, while the head of the Stanley Crane (A. paradisea) appears to be swollen, so abundantly is it feathered. The crowned cranes, although bare-cheeked, have double crests, the two parts of which have been respectively compared to a pen-wiper and a bunch of toothpicks!

Among the guinea-fowls, several species are crested, while others, as, for example, the domestic one, are bare-headed. Now, on the theory of evolution, by accumulation of minute variations, phenomena such as these are difficult of explanation; but, on the assumption that a slight rearrangement of the biological atoms in the molecule may produce very diverse results, as we see in the case of chemical molecules, and of seasonally dimorphic butterflies, there is no particular ground for surprise at such a phenomenon.

In this connection we may cite the significant fact, so well known to canary breeders, that two crested birds when mated tend to produce a bald-headed one.

If the colour of any part of an organism be due to the internal arrangement of the constituent parts of the biological molecule from which it is derived, we should expect any rearrangement of the component parts to produce quite a different colour. In other words, we should expect occasionally to see colour-mutations. These are precisely what we do see. Similarly, if the scheme of colouring of an organism be due to a certain grouping of biological molecules, we should expect the same scheme of colouring to occur in organisms which are not nearly related. This, too, we observe in nature.

Many of the phenomena of mimicry, and all the cases which we have cited as pseudo-mimicry, seem to us to be referable to this.

Magpie Colouring

Take, for example, the magpie colouration in birds—that is to say, a scheme of colouring in which the body is white, and head, wings, and tail black. This occurs in the following birds belonging to the most diverse groups:—

The Magpie.

The Magpie Tanager (Cissopis leveriana).

The Magpie Robin (Copsychus saularis), cock only; in the hen the black is replaced by brownish grey.

The Pied Honeyeater (Entomophila picata).

The Chaplain Crow (white-bodied form of the hoodie crow).

The New Ireland Swallow Shrike (Artamus insignis).

The Magpie Goose (Anseranas melanoleucus).

Combinations of this kind, in which the black is replaced by brown or grey, are excessively rare.

On the other hand, we see in several birds the combination in which the white is replaced by yellow:—

The Common Troupial (Icterus vulgaris).

The Black-headed Oriole (Oriolus melano cephalus).

The Black-and-yellow Grosbeak, male only.

What we may call imperfect magpie colouration, i.e. where the head becomes white, occurs in several species of birds. The head of a black species sometimes becomes white as a mutation; in the domestic Muscovy duck, for example, an individual is sometimes produced having a white head, although the black of the remainder of the plumage remains unchanged.

As examples of this scheme of colouration we may cite—

Black-and-white Fruit Pigeons (MyristicivorÆ).

Several Gannets (Sula capensis, S. serrator, etc.)

Swallow-tailed Kite (Elanoides furcatus).

Several Storks (Euxenura maguari, Anastomus oscitans, Pseudotantalus cinereus).

Moreover, a common variety of the barn-door fowl has also a white body and black primaries and tail, showing that this scheme of colour may arise as a mutation.

A further elimination of black in the tail and body leads us to white birds with more or less black wings:—

White Storks (Ciconia alba, C. boyciana, and Euxenura maguari).

The White Crane (Grus leucogeranus).

The Snow Geese (Chen nivalis, C. rossi).

The Common Gannet (Sula bassana).

The White Buzzard (Leucopternis).

The Scavenger Vultures (Neophron).

A recurring combination in mammals is black, with a white marking on the breast.

Most of the bears, even young brown bears, show a tendency to this. It is also found in the Tasmanian devil, and in varieties of our domestic cats, rats, and dogs; also in the domestic duck.

The white-spotted pelage, not uncommon in deer, especially fawns, is curiously repeated in the Australian carnivorous marsupials, known as Native Cats (Dasyurus).

In domestic animals we frequently find the following localisation of white—white socks, collar, breast, and muzzle. The arrangement occurs in cats, dogs, rabbits, guinea-pigs and mice, also in the horse and pig, but without the collar. The arrangement is not seen in goats, cattle, or sheep, nor in wild animals of any kind. This would lead to the conclusion that the combination is correlated with some character unfavourable to survival under natural conditions.

Many variations which frequently occur among both wild and domestic animals do not persist in nature.

Albinos

As instances of such variations we may mention pure albino forms, that is to say those in which pigment does not occur in the eyes.

It is easy to see why this variation is not allowed to persist in nature. Its possessors are handicapped by bad eyesight, and so have no chance of surviving in the struggle for existence. It is thus that natural selection acts. On the other hand, white species with pigmented eyes are fairly numerous. These enjoy normal eyesight, but labour under the disadvantage of being easily seen by their foes. Hence we find that white species generally either occur in a snowy habitat, or are powerful and both able and ready to defend themselves. In this connection it is interesting to notice that in New Zealand all birds, whether introduced or indigenous, are particularly liable to albinism. Owing to the fewness of their enemies these albinistic forms are able to persist.

A variation, or rather a mutation, that frequently occurs among domesticated birds, but which is seen in very few wild species, is that which takes the form of white primary feathers on the wing. This variation must often occur in nature, but it rarely establishes itself, apparently because white feathers do not resist wear so well as coloured ones do.

Biological Molecules and Colour

Black-and-yellow colouration occurs in several widely separated species of birds. The arrangement of the two colours follows to some extent the same rules as the black-and-white combination.

Several birds have a yellow body with black head, wings, and tail, such as—

The Black-headed Oriole (Oriolus melanocephalus).

The Black-and-Yellow Grosbeaks (Pycnorhamphus icteroides, P. affinis) (cock).

The Common Troupial (Icterus vulgaris).

In others the black on the head is nearly or quite suppressed, that on the tail remaining to a greater or less extent; such are—

The Golden Orioles (Oriolus galbula, O. kundoo, etc.).

Several species of Icterus.

Several fly-catchers of the genus Piezorhynchus (males only).

BRAZILIAN TROUPIAL

This species (Icterus vulgaris) is that most frequently seen in captivity; the pattern of colour is found in several other allied forms.

INDIAN BLACK-HEADED ORIOLE

Several other orioles besides this (O. melanocephalus) have the black head.

We have said sufficient to show that certain combinations of colours recur in nature in species which are neither nearly related to one another nor subjected to similar environment. For such phenomena it is difficult, if not impossible, to account on the theory that natural selection, acting on minute variations, is responsible for all the varied colouring of the animal kingdom. The facts, however, are in accordance with the supposition that the organism is the result of the growth and development of a number of units or biological molecules which exist in the fertilised egg.

If there be any truth in the supposition, the colouration of every animal must be due to the development of one or more of these molecules. Colouration may be expression of the arrangement of all the molecules in the fertilised egg, or it may be due to the development of a number of molecules whose function is to determine the colouring of an organism, or it may be the result of the development of one such molecule, which perhaps splits up in such a way that a portion attaches itself to each of the other molecules.

But it is idle to speculate on this point. As we have already insisted, the tendency to build up elaborate theories on very slender foundations is a too frequent failing of zoologists. We desire merely to emphasise the fact that the phenomena of animal colouration almost force us to the conclusion that the colouring of each organism is the result of the development of a number of units.

It may be objected that, if this be the case, the number of the units which contribute to the colour of any organism must be exceedingly large, since we see in nature an almost limitless number of different schemes of colouring. If the colour of each animal be the result of the development of a few units, it might be thought, firstly, that the diversity of schemes of colouration which we observe in nature could not possibly occur; and secondly, that, under such circumstances, the colour pattern of a bird or beast should be of the nature of a mosaic, each colour being sharply defined and separated from every other colour, instead of the colours shading one into the other, as is so frequently the case.

Such objections would be based on a misconception as to the nature of the units which combine to produce the colouration of an organism. These units show themselves as centres of development of colour, as points from which the colour or colouring they represent spreads, until it meets and mingles with other patches of colour which are being developed from other centres. The colour produced at one centre may spread more rapidly than that which forms at another; this, of course, will result in a preponderance in the organism of the colour which is produced at the former centre.

Further, we must bear in mind that the development of each colour-producing unit is largely affected by conditions external to it, as we shall see when dealing with Sexual Dimorphism.

More than one naturalist, who has paid careful attention to the subject of animal colouration, has perceived that through the apparently endless diversity of the colouring of organisms something like order runs.

Mr Tylor Quoted

Over thirty years ago Mr Alfred Tylor called attention to this important fact. That observer, whose views met with the approval of Wallace, was of opinion that colour follows structure, and that in a many-hued animal it changes at points where the function changes.

“If,” writes Mr Tylor, “we take highly decorated species—that is, animals marked by alternate dark or light bands or spots, such as the zebra, some deer, or the carnivora, we find, first, that the region of the spinal column is marked by a dark stripe; secondly, that the regions of the appendages, or limbs, are differently marked; thirdly, that the flanks are striped or spotted, along or between the regions of the lines of the ribs; fourthly, that the shoulder and hip regions are marked by curved lines; fifthly, that the pattern changes, and the direction of the lines, or spots, at the head, neck, and every joint of the limbs; and, lastly, that the tips of the ears, nose, tail, and feet, and the eyes are emphasised in colour.”

More recently Mr J. Lewis Bonhote has devoted much attention to this important subject. The results of his researches are summarised on page 185 of vol. xxix. of the Proceedings of the LinnÆan Society, and on page 258 of the Proceedings of the Fourth International Ornithological Congress, 1905. Mr Bonhote states that the presence or absence of colour tends almost invariably to make its appearance, first of all, on certain definite tracts, common to mammals and birds alike, which he calls poecilomeres.

Poecilomeres

“Poecilomeres,” he writes, “are situated on the following parts, viz., chin, malar stripe, maxillary stripe, a spot above and slightly in front of the eye, a spot below or slightly behind the eye, the ear, crown of the head, occiput, fore-end of sternum, vent, rump, thighs, wrist, shoulders (above and below).

“Now, there is hardly any species of bird on which one or more of these poecilomeres is not ‘picked out’ (to use a painter’s expression) in some colour different from that of the surrounding parts, and, in fact, most of the so-called recognition or protective markings will be found on these patches.

“On the other hand, among many species the differentiation of colour on the poecilomeres is not so conspicuous as to attract the eye or to serve in any way for protection or mimicry, yet we still find them marked by differences of colour so slight that, unless especially looked for, they would never be noticed.

“Or, again, some species occasionally, but not invariably, show a few white feathers on certain parts of their body, and, when such is the case, it will be found that these white feathers appear on the poecilomeres. . . . There is hardly a species in which examples of these poecilomeres may not be found. . . . The Kingfisher (Alcedo ispida) shows the various head poecilomeres very clearly, and as examples of inconspicuous differences on these tracts, the rump of the hen sparrow (Passer domesticus) and hen chaffinch (Fringilla coelebs), the malar stripe and dark ear-patch of the hen Yellow Bunting (Emberiza citrinella), and the dark ante-orbital patch of the Barn Owl (Strix flammea) are familiar examples. And, lastly, as an instance of the class where a few white feathers frequently, but not invariably, appear, the young of the cuckoo (Cuculus canorus) forms a good example.

“These spots may, however, appear in a transitory manner, as, for instance, where a change of plumage (not necessarily moult) is occurring.”

As an instance of this, Bonhote cites the case of a young male Shoveler (Spatula clypeata), “in which the metallic colour on the head first showed itself on the post-orbital and auricular poecilomeres, gradually meeting and joining up across the head with the crown and occipital poecilomeres, and then finally spreading forwards. And it may be well to note that the joining up of the auricular and post-orbital poecilomeres formed a metallic patch similar in size and position to that found in the male Teal (Querquedula crecca), and, further, in the last stage, when the whole head, except the portion round the beak, was metallic, the markings are similar to those found permanently in the hen Scaup (Fuligula marila).

“Now, these resemblances taking place in the normal pure-bred wild shoveler, the question of reversion does not come in, and no one would suppose these resemblances due to anything more than transitional variation, and it is the object of this portion of the paper to show that variation in colour follows along definite lines.”

Biological Molecules

Mr Bonhote continues: “As a further illustration of how widely spread these lines are throughout the mammalian and avian kingdoms, we may note the assumption of the brown head in the case of the Black-headed Gull (Larus ridibundus), which invariably follows each year on lines similar to those related in the case of the shoveler, and . . . the method by which, on the approach of winter, the stoat assumes his white dress, is (although the change is from brown to white) again conducted along precisely similar lines.” Mr Bonhote argues with great force that, as the process occurs in two animals so widely separated, the fundamental cause must be a deep-seated one. There can be no doubt that these poecilomeres of Bonhote are connected with our biological molecules. Each of these poecilomeres is the result of the development of one of these unit characters; each is to be regarded as the centre of activity, the sphere of influence of a biological molecule, or the portion of one, which controls the colouring of a definite region of the organism. In the case of creatures which display the same colour throughout, these molecules all give rise to the same kind of colouring; in the case of animals which display a variety of colours and markings the various molecules give origin to various colours. But we must bear in mind that the final colour to which each colour-producing molecule gives rise depends to some extent on circumstances other than the constitution of the molecule. Thus it is that the young in most organisms differ in colour and marking from the adults. On this also depends the phenomena of seasonal and sexual dimorphism. The same colour-producing molecule may give rise to one colour under one set of conditions and to a totally different colour under another set of conditions.

It is a significant fact that under abnormal conditions the feathers of birds tend to disappear precisely on those spots where the poecilomeres of Bonhote occur.

Thus in a sickly cage bird the feathers frequently show a tendency to fall off on the following spots: crown of head, lores, jaws, head generally, rump, vent and thighs.

Many wild birds—as, for example, the cranes—display patches of naked skin on the head, and these are usually situated on poecilomeres. Similarly, natural excessive developments of plumage tend to occur on the poecilomeres, or, rather, the spots characterised by poecilomeres—for example, the train of the peacock. Loral plumage, it is true, is seldom long, but is often of a peculiar nature.

Colour mutations tend to occur on the poecilomeres. Thus it is that these poecilomeres often form the distinctive characters and markings of allied species. This is precisely what we should expect if the poecilomeres correspond to biological molecules and mutations are the result of the rearrangement of the constituent parts of these molecules.

Still more significant is the fact that the colour-markings in hybrids tend to follow poecilomeres.

Bonhote has performed a large number of experiments in hybridising ducks. Some of his hybrids were produced from three pure ancestors, as, for example, the pintail, the spotbill, and the mallard; others from two ancestors. Some of these hybrids were crossed with other hybrids, and others with the parent forms, hence Bonhote secured a number of hybrids, each of which had a distinctive appearance; but all the variations appearing among the hybrids were found to start on one or more of the poecilomeres.

Certain of the hybrids showed a resemblance to one or other of the parent species, others were unlike either parent, and resembled either no known species or species other than their parents.

When a hybrid shows a resemblance to a species other than that to which either parent belongs, it is said to exhibit the phenomenon of atavism or reversion,—the individual is supposed to have been “thrown back” to an ancestral form.

The true explanation of the phenomenon would seem to be that, as the result of the crossing, biological molecules in the fertilised egg have been formed which, on development, give rise to combinations of colour like those seen in other species.

Thus the phenomena of “mimicry” and “reversion” are, we believe, due to the fact that in the fertilised egg of both the pattern and its copy a similar arrangement of biological molecules obtains. If we regard the sexual act as resembling in many respects a chemical synthesis, the phenomenon need not surprise us.

To sum up, the observed facts of animal colouration seem to indicate that there are in each organism some twelve or thirteen centres of colouring, which we suggest may correspond with portions of the fertilised egg. From each of these centres the colour develops and spreads, so that every part of the organism is eventually coloured. These centres of colouring are not altogether independent of one another. Sometimes they all give rise to the same hue, in which case we have a uniformly-coloured organism, such as the raven. More often from some one colour develops, and from others another colour; if these two colours happen to be black and white, the result is a pied organism, which displays a definite pattern due to the correlation of the various colour-producing biological molecules.

Thus it occasionally happens that two widely different organisms exhibit very similar markings, and therefore resemble one another. When this resemblance is believed to be of advantage to one or other of the similarly-coloured species, naturalists call it mimicry, and assert that the likeness is due to the action of natural selection; but where neither organism can profit by the resemblance, zoologists make no attempt to explain it. What we suggest is that the colouration of an animal depends upon the structure, or, at any rate, the nature, of the parts of the egg which produce these centres of colour. But this is not by any means the only cause that determines the colouration of the organism. If it were, young creatures in their first plumage would invariably resemble the parents, the two sexes would always be alike, and there would be no such phenomenon as seasonal dimorphism.

As a matter of fact, the portions of the egg (we call them, for the sake of clearness, colour-producing biological molecules) which give rise to the poecilomeres exhibit themselves merely in the shape of tendencies; the ultimate form the colouring will take depends to a large extent upon other and extraneous circumstances, such as the secretion of hormones.

Thus it is that organisms seem to display an almost endless diversity of colouration. But beneath all this diversity we see something like order. It occasionally happens (why, we do not know) that one, or more, of the biological molecules which make up the nucleus of the fertilised ovum becomes altered in the sexual act, with the result that a discontinuous variation or mutation appears in the resulting organism. The mutation may be a favourable one, or one which does not affect in any way the chances of an organism in the struggle for existence, or an unfavourable one. In the last of the three cases the organism will perish early and not leave behind any offspring exhibiting its peculiarity.

It is thus that natural selection acts. Natural selection weeds out relentlessly all organisms which display unfavourable variations. It is thus obvious that many species may, and we believe do, exist which possess characters of no direct utility to them, or even slightly harmful ones. For this reason Wallace and his followers fail in their attempts to prove that every patch of colour in every organism is of direct utility. Natural selection has to take an animal as it finds it—the good with the bad. If an organism as a whole is not wanting—that is to say, if it is able to hold its own against other organisms, and is fitted to fill any place in nature—that organism will probably survive, although it may be defective in many respects. As its name implies, natural selection is a mere selecting agency. It has to choose from what is presented to it. It is not, as many seem to think, a manufacturer or inducer of variations. Natural selection can no more make an animal vary in any given direction than the human breeder can. Its power is limited to the destroying of all variations which do not pass the test prescribed by it.

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