(1859 AND LATER)

Period V.

We do not attempt to characterise our last period, nor to describe its biological achievement. It seems better to devote the whole of our scanty space to the scientific careers of Darwin and Pasteur, in which so much past effort culminated, and from which so much progress was to spring.

Darwin on the Origin of Species.

Setting aside as superfluous and we might say impossible, under our conditions of space, all attempt to restate the evidence on which Darwin based his great argument, we shall here try to show that the Origin of Species shed a new light upon many biological facts, combined many partial truths into one consistent theory, and gave a great stimulus to further inquiry.

1. Classification and Affinity.—The sixteenth-century herbalists and still earlier writers (see p. 17) recognised a property of affinity, by which plants were associated in natural groups. Bock (1546) tried to bring together all plants which are related (verwandt) to one another, but similarity of any kind was with him a proof of affinity; it did not shock him to place the dead nettles next to the stinging nettles. L'Obel gave names to several families of flowering plants which are still admitted as natural. Ray spoke of the affinity (cognatio) between plants, and his affinity was a thing not to be violated for the sake of practical convenience or logical rules, but he was unable to explain what he meant by it. LinnÆus tried to illustrate affinity between plants by contiguous provinces on a map, a better metaphor than the linear scale, for the scale can only express affinity on two sides, while the map can express affinity on many. His practical experience of classification taught him a truth, shocking at first sight to the logician[44]—viz., that the characters which serve for the definition of one genus may be useless for the definition of the next, and he laid it down that the characters do not make the genus, but the genus the characters. After LinnÆus we find for a long time no advance in the philosophy of natural classification. Cuvier (1816) is even retrograde, for he sets aside the maxims of LinnÆus, maintains that adaptive characters (characters closely related to the conditions of life) are relatively constant, and that large groups should be defined by characters drawn from organs of great physiological importance. These decisions of his are repudiated by later naturalists.

The key to the affinity puzzle which had so long baffled thinking naturalists was at last supplied by Darwin, who explained that "the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, all true classification being genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike."[45]

Natural groups, large or small, result from the long-continued operation of divergence, the survival of some, and the extinction of others; they are to be respected as facts; they are not created by definitions, which only serve to indicate and remind; any character, however trifling, will suffice, if only it is constant and distinctive.

The conflict between natural classification and logic is apparent only. Logicians say that in classifying books, for instance, you may take any property you please, subject, size, etc., as the basis of your arrangement, but having made your choice, you must adhere to it for all divisions of the same rank. Naturalists seem to say something different, for they are agreed that what they call "single-character classifications," in which one property is adhered to throughout, are unnatural. The fact is that a natural classification always rests upon one and the same property—viz. affinity, i.e. relative nearness of descent from some common ancestor. Every natural classification, like every logical classification, proceeds upon a single basis, and the failure of the single-character classifications is due to their displacing affinity by some definition.

The effect of the Origin of Species upon zoological and botanical systems has been revolutionary. Furnished with a new and intelligible meaning of the word natural, and with new criteria of naturalness, systematists have during the last fifty years worked hard to create classifications which admit of being thrown into the form of genealogical trees. Wide gaps in the geological history of life render the task difficult beyond expression, but much has already been accomplished. Newly discovered forms (especially the fossil ArchÆopteryx and the Cycadofilices) and more fully investigated forms, far too numerous to be specified, have established links between groups which formerly seemed to be wholly independent. Unnatural assemblages based on pre-determined characters (Radiates, Entozoa, Birds of prey, etc.) have been replaced by groups which are at least possible on evolutionary principles. Almost every working naturalist will admit that the progress of zoological and botanical system during the last two generations has done much to fortify the Darwinian position.

2. Embryology.—Baer in 1828 was possessed of all the embryological facts which Darwin used in support of his theory of evolution; in particular, he was well acquainted with the most striking fact of all—viz., the presence in embryo mammals and birds of a series of paired clefts along the sides of the neck, between which run vessels arranged as in gill-breathing vertebrates. The vessels had been figured by Malpighi; the clefts had been discovered by Rathke, who had no hesitation in calling them gill-clefts and the vessels gill-arches. Nor had Baer, who nevertheless to the end of his long life refused to accept the one explanation which gives meaning to the facts—viz., that remote progenitors of mammals and birds breathed by gills. Few embryologists have since felt such a scruple. The adaptation to gill-breathing is obvious; is gill-breathing now practised by any mammal or bird? Certainly not. Is it destined to be practised by their descendants at some future time? To say nothing of the danger of putting forth any such prophecy, it involves all the consequences of descent with modification. The opponent of evolution may as well admit at once that the gill-breathing was practised in time past. As an example of the same kind taken from plants, we may quote the trifoliate leaves of the furze-seedling, which, though absent from the full-grown furze, are frequent in the family (LeguminosÆ) to which it belongs. The general similarity of vertebrate embryos, of insect-embryos and of dicotyledonous seedlings, is also worthy of note. We may suppose that early embryos, being largely or wholly dependent on food supplied by the parent, and perhaps protected by the parent as well, escape the pressure of the struggle for existence, and are often not urgently impelled to produce adaptations of their own. In these circumstances it is intelligible that features inherited from remote ancestors should persist. If, however, early independence is demanded by the conditions of life, the embryo may develop temporary adaptations, wanting in the parent and in embryos of allied groups. Larval adaptation is as much a part of the economy of nature as the retention of ancestral structures which have been lost by the adult.

3. Morphology.—Let us next consider the light which the Origin of Species throws upon homologous parts. No example will serve our purpose better than the very familiar one of the fore-limbs of different vertebrates, the arm and hand of man, the wing of the bat, the wing of the bird, the pectoral fin of the fish, and the paddle of the whale. These limbs, adapted for actions so diverse as grasping, running, flying, and swimming, nevertheless exhibit a common plan, evident at a glance, except in the pectoral fin of the fish. But why a common plan? Of what advantage is it to an animal that its wing, paddle, or hand should reproduce the general plan of a fore-foot? Why should the digits of the land vertebrates never exceed five? Why should the thumb never have more than two free joints? It would be hard to find a satisfactory answer to these questions in any book earlier than the Origin of Species; no student of the Origin of Species finds any difficulty in answering them all. The common plan has been transmitted from type to type by inheritance, and its features are derived from an unknown common ancestor.

The new conception, that structures inherited from remote ancestors may be incessantly modified by the conditions of life and by mutual competition, is the key to the chief problems of morphology. No limited collection of examples can substantiate so wide a proposition as this. Those who have made themselves familiar with old text-books of comparative anatomy will recollect how dry, or else how inconclusive, was pre-evolutionary morphology, how vague were the references to some ideal archetype, or to climate, or to the ancient conditions of the earth's surface; how often exclamations of admiration for the marvels of nature or Providence were substituted for clear explanations. Cuvier, it is true, was both precise and reasonable; but how little he was in a position to explain! His "empirical" comparative anatomy could throw no direct light upon origins or transformations; his "rational" comparative anatomy was practicable only in a few easy cases.

4. Geographical Distribution.—The facts of distribution were handled in the Origin of Species with great originality. It was shown that they support, and indeed require, some doctrine of organic evolution. The succession in the same area of the same types—armadillos succeeding armadillos in South America, marsupials succeeding marsupials in Australia—was enough of itself to render independent creation highly improbable. This was not all. Darwin's mind being charged with facts and reasonings, the accumulations of many years of travel and meditation, he sketched in rapid outline conclusions which have given a new form to the distribution question. The subject had hitherto been treated by collecting masses of facts and interpreting them by recent physical geography; Darwin showed that the history of the continents and islands may be far more influential than soil, elevation, or climate.

The scientific discussion of the facts of distribution is as old as the sixteenth century, when L'Obel pointed out that the mountain plants of warm countries descend to low levels in the north. LinnÆus remarked that fresh-water plants and alpine plants are often cosmopolitan. Another early and well-founded generalisation is the statement of LinnÆus that the plants common to the old and the new world are all of northern range. Buffon made the same remark about the animals, and offered the probable explanation—viz., that since the two great land-masses approach one another only in high latitudes, it is only there that animals have been able to cross from one to the other.

In the nineteenth century theories involving prodigious changes of land and sea were much in the minds of naturalists. Darwin lost his temper (a rare thing with him) over the land-bridges, hundreds, or even thousands, of miles long, which were created in order to explain trifling correspondences in the population of distant countries. A belief in the comparative stability of the great continents and oceans has since prevailed, and it is now recognised that the means of dispersal of species are greater than was once supposed.

The discovery, about the year 1846, of the marks of ancient glaciers in all parts of northern Europe, and the acceptance of an Ice Age, had a still greater influence upon the teaching of naturalists. Edward Forbes[46] put forth a glacial theory to account for the present distribution of plants of northern origin. Glacial cold, he maintained, had driven the arctic flora far southward. When more genial conditions returned, most of the northern plants retreated towards the Pole, but some climbed the mountains and gave rise to an isolated alpine flora. Darwin, whose unpublished manuscripts had anticipated Forbes's theory, believed that the whole earth became chilled during the Ice Age, and that the fauna and flora of the temperate zone reached the tropics. His argument, which is contained in chap. xi. of the Origin of Species, is now generally accepted in principle, though opinions differ on many points of detail. Some think that he extended too widely the effects of glacial cold, exaggerated the resemblance of the arctic and alpine fauna and flora, and attributed the extinction of the northern species in the intermediate plains too exclusively to climatic causes.

One paragraph in the extremely condensed discussion on geographical distribution which we find in the Origin of Species calls attention to the dominance of forms of life "generated in the larger areas and more efficient workshops[47] of the north." The power which inhabitants of the great northern land-mass of the old world, and in a less degree those of North America, possess, and have long possessed, of driving out the inhabitants of the southern continents is one of the most important factors in the peopling of the earth with new races of land-plants and land-animals. Races of men, modes of civilisation, religious faiths, all follow the same rule, which has no doubt prevailed ever since land came to predominate in the northern hemisphere and water in the southern hemisphere. In the life of the sea and the fresh waters no dominance of northern forms has been detected.

5. PalÆontology.—We must not claim for Darwin more than a modest share in the vast extension of palÆontological knowledge which the last fifty years have created. A profusion of new materials has been acquired by the diligence of collectors working on a scale previously unattempted. But though the accumulation of materials is the work of others, the interpretation has been guided by the principles of Darwin. The evolution of the horse has now been so fully worked out that it would bear the whole weight of a doctrine of descent with modification, though it could not by itself reveal the process by which the modification had been effected.

Darwin on Adaptations.—The adaptation of living things to their surroundings has always been a favourite branch of natural history, underrated only by those whose studies are little calculated to inflame the curiosity. Many eminent naturalists have made the interpretation of natural contrivances their chief aim. Darwin equalled the best of his predecessors in accuracy, range, and ingenuity, while he surpassed them all in candour. No one has done so much to vindicate the study of adaptations from all suspicion of triviality, for no one before him had seen so clearly how all new species arise by adaptation of pre-existing ones. It is by adaptation that new forms of life arise; it is inheritance which preserves old ones.

Socrates, Swammerdam, and Paley had drawn from the adaptations of nature proofs of the omnipotence and beneficence of the Creator. Darwin, while admitting that every organism is exquisitely adapted to its own mode of life, believed that the adaptations have been perfected by slow degrees, and that they cannot be proved to have been consciously devised. This interpretation deprives the theologian of valued arguments, but at the same time rids him of difficulties. Even before Darwin's day some few natural theologians had the courage to bring forward instances of the harshness of nature. Kirby and Spence[48] thought that no injustice was done to certain predatory insects by comparing them to devils. Others blessed the mercy of heaven, which, after creating noxious animals, created others to keep them in check. Darwin, when reflecting upon the odious instincts which urge the young cuckoo to eject its foster-brothers, some species of ants to enslave others, and a multitude of ichneumons to lay their eggs in the bodies of live caterpillars, found it a relief to be able to shift the responsibility to an unconscious natural process.[49]

In his autobiography Darwin remarks that he had thought it almost useless to endeavour to prove by indirect evidence that species had been modified until he was able to show how the adaptations could be explained. Some of them alarmed him by their difficulty; to suppose that the eye, with all its inimitable adjustments, had been formed by an unconscious natural process seemed to him absurd until he had traced a good many intermediate steps between the mere colour-spot and the eye of the eagle. He writes to Asa Gray (September 5, 1857) that the facts which had done most to keep him scientifically orthodox were facts of adaptation, the pollen-masses of Asclepias, the mistletoe with its pollen carried by insects and its seeds by birds, the woodpecker exquisitely fitted by feet, tail, beak, and tongue to climb trees and capture insects.

The student of adaptations has no longer a moral thesis to maintain; he tries to understand how a contrivance acts, what advantage it confers upon its possessor, and by what steps it was perfected. The minute variations of species are as capricious as the form of the stones which accumulate at the foot of a precipice; natural selection turns fortuitous variations to account for the advantage of the species as a builder might turn to account the shapes of the stones. Man himself can employ variations for frivolous or even base purposes, as when he produces toy-spaniels or bull-dogs.[50] The adjustments of organic structures often move our wonder by their perfection. One reason why they so far exceed the adjustments made by wind, frost, or moving water is that the process has been so protracted; in a worm or an insect we see the last stage of an adaptation which has been continuously at work for untold geological periods. Another reason is that the thing adapted is alive, sensitive, and capable of responding to the subtlest imaginable influences.

Darwinism and Non-biological Studies.—The theory of organic evolution has already produced a visible effect upon non-biological studies. Bagehot has applied Darwinian principles to the interpretation of history and politics. Philologists recognise a process very like that of natural selection in the modification of words. The usages of language are inherited from generation to generation; one idiom competes with another, that persisting which best suits the temper or the convenience of the nation. Philology has like zoology its chains of descent, its breeds or dialects, its species or languages, its fossils (dead languages), its dominant and declining forms, its vestiges (such as letters, still retained, though no longer sounded). Psychology is already in part experimental and evolutionary, and seems as if it would attach itself more and more closely to physiology, detaching itself in the same measure from metaphysics. The change may be attributed to two growing convictions: (1) That the experimental method is more trustworthy than the speculative; and (2) that the mind of man is not a thing apart, but an enhanced form of powers manifest in the lower animals. Sociology finds its most practicable and its most urgent sphere of work in the problems of selection and race, which are naturally examined in the light of Darwinian principles. The new study of Comparative Religion aims at the impartial examination of all forms of religious experience, and is evolutionary in proportion as it is scientific. One of its conclusions, by no means universally accepted as yet, is the recognition of conscience as "the organised result of the social experiences of many generations" (Galton). Comparative Religion can already show in outline how by slow degrees magical rites passed into polytheistic worship, how polytheism became simplified and elevated, and how ethical motives at length became influential if not predominant.

Pasteur's Experimental Study of Microbes.

The same difficulty arises with Pasteur as with Darwin; his life-work has already been described often and well. Readers unversed in science have only to turn to the Vie de Pasteur, written by his son-in-law, Vallery-Radot, to find a luminous account, giving just so much detail as makes the discoveries intelligible and interesting. If shorter sketches are demanded, they exist. We must therefore above all things be brief, and content ourselves with reminding the reader of facts which, in spite of their recent date, are as well known as anything in the history of science.

Chemists will claim Pasteur as one of their number, and we do not dispute the claim. Trained in experimental methods by the chemical laboratory, he devoted his best powers to the study of living things, and, without ceasing to be a chemist, became one of the greatest of biologists.

Pasteur's chief work was of course the experimental investigation of living particles which float in the air—what we may call live dust. Before his day such particles had been seen, named, and classified; some few had been studied in their action and effects. Most of them are plants of low grade, simplified to the last point for the sake of minuteness, on which their ready dispersal depends.

Yeast.—Van Helmont, early in the seventeenth century, when the microscope had not yet become an instrument of research, attempted to investigate the fermentation of beer, and made acquaintance with the properties of the gas which is evolved, his gas silvestre, which was afterwards called fixed air, or carbonic acid. Leeuwenhoek about 1680 examined yeast by his microscopes, and discovered that it is made up of globules which often cohere, and that these globules give off bubbles of gas. Then comes a long interval, during which nothing was done to elucidate the process of fermentation. It was not till 1837 that Caignard-Latour and Schwann, independently of each other, showed that yeast-globules multiply by budding, and are therefore to be set down as living things, probably plants of a simple kind. Twenty years more passed without sensible progress; during this time chemists were striving to prove that the alcohol was produced by contact-action, and that the globules were of no practical importance. By the year 1860 Pasteur was engaged upon the problem. It is well known that he arrived at a firm conviction that living yeast-cells are essential to the production of alcohol. It has since been discovered that the enzyme (unorganised ferment of older writers) secreted by living yeast-cells can change sugar into alcohol after the cells themselves have been destroyed, and that other plants besides yeast-cells secrete the same enzyme when deprived of oxygen.

Bacteria.—Another and even more important chapter in the history of air-wafted organisms was opened by the indefatigable Leeuwenhoek. In 1683 he wrote a letter to the Royal Society which makes mention for the first time of bacteria, which he found upon his own teeth, and described as minute rods; some of them moved with surprising agility. For nearly two hundred years little more was done. A few bacteria were named and classified, and there the matter rested until Schwann proved experimentally that putrefaction is just as much the work of living microscopic organisms as alcoholic fermentation. In 1857 and the following years Pasteur not only confirmed the work of Schwann, which had been received by the majority of chemists with distrust, but went on to show that the lactic, butyric, and ammoniacal fermentations also depend upon the activity of bacteria. The happy thought struck him that they might be studied alive—a possibility which he soon realised in practice, and upon which the new science of bacteriology largely rests. From about the year 1873 he began to occupy himself seriously with contagion, which he suspected to be connected with specific aerial germs. Davaine and others had years before observed in the blood of sheep and cattle which had died of "charbon" (anthrax) minute "bÂtonnets" (bacilli). Pasteur's published results induced Davaine to ask whether his "bÂtonnets" might not be the cause of "charbon." Again, it was Pasteur's results which induced Lister to make experiments in the field of antiseptic surgery. Pasteur wasted no time upon the curiosities of bacterial life. His first studies on fermentation suggested that specific diseases may be propagated by microscopic germs, and that such cases of spontaneous generation as had hitherto escaped refutation might be explained by the access of live dust. The identification and biological history of the organisms interested him only as a step towards sure methods of controlling, and, if necessary, destroying, them; of mitigating their virulence by inoculation; of rendering animals immune against them; or of stamping out the disease by isolation. All this is happily too well known for repetition here. The story, with its many dramatic incidents, can be read in the pages of Vallery-Radot.

Hardly less important than the bacteria which destroy life or endanger the products of human industry are the beneficent forms, some of which have in all ages co-operated with man, while others can only be employed by those who possess knowledge and skill. None are so important to our welfare as the bacteria which renew the fertility of the soil. But for the soil-bacteria farm-yard manure would be useless to the crop, for it is they which render it fit for assimilation. Now the bacteria of the soil have their natural enemies, the most mischievous being certain Protozoa, such as Amoeba and its kindred. As soon as this fact was grasped, likely remedies were thought of; indeed, one remedy was suggested without any guidance from theory by a vine-grower of Alsace, who treated his soil with carbon disulphide to destroy phylloxera, and found that in so doing he had notably enhanced its fertility. Heating to the temperature of boiling water destroys the soil-protozoa and at the same time the bulk of the soil-bacteria. The bacteria, however, soon multiply more than ever by reason of the absence of their enemies, and a soil cleared of protozoa yields for a few years appreciably richer crops. Of other useful bacteria the briefest notice must suffice. Wine, beer, cheese, and tobacco owe to certain of them distinct flavours, for which the customer is willing to pay high. Leather in certain stages of manufacture, indigo, and woad require the access of other forms. If we also bear in mind the part which yeast plays in the every-day manufacture of bread, wine, and beer, and the part which the vinegar-mould plays in the manufacture of acetic acid, we shall get some notion of the industrial importance of the various micro-organisms. Not a little of the control which we exercise over them we owe directly or indirectly to Pasteur.

The career of Pasteur exhibits a striking unity. His first research, which dealt with a subject so remote from the ordinary studies of the biologist as the crystalline forms of tartrates, made him acquainted with activities, hitherto unsuspected, of minute forms of life. The hope of aiding the industries of Lille, Orleans, and France kept him long engaged upon ferments. If he turned aside to examine the superstition of spontaneous generation, it was to protect his methods from misconstruction. An apparent break in his programme of work was forced upon him by the silkworm pestilence. It proved to be no real break, for pÉbrine and flacherie were both bacterial diseases. At a comparatively early date (1863) he wrote that his chief ambition was to throw light on the spread of contagious diseases; he could not then foresee that he was destined, not only to elucidate, but in a measure to control them. Around his tomb are inscribed words, each of which commemorates a signal service to his fellow-men: "1848, Molecular dissymetry. 1857, Fermentations. 1862, Spontaneous generation. 1863, Studies of wine. 1865, Silkworm diseases. 1871, Studies on beer. 1877, Contagious diseases of animals. 1880, Vaccination against contagious diseases. 1885, Prevention of hydrophobia." These manifold researches form a continuous chain, each being linked to what precedes and follows. The devotion by which all were inspired, beginning with devotion to science and the fatherland, ended by embracing all mankind.

Biology, which in the sixteenth century sent out only a few feeble shoots, has now become a mighty tree with innumerable fruit-laden branches. The vigour of its latest outgrowths encourages confident hopes of future expansion.

---