Climatic Varieties
In this chapter we will examine certain cases which illustrate phenomena comparable with those just considered, though as I have already indicated, they form to some extent a special group. The outstanding fact that emerges prominently from the study of the local forms is that when two definite types, nearly allied, and capable of interbreeding with production of fertile offspring, meet together in the region where their distributions overlap, though intergrades are habitually found, there is no normally or uniformly intermediate population occupying the area of intergradation. Such phenomena as these must, I think, be admitted to have great weight in any attempt to construct a theory of evolution. True we must hesitate in asserting their positive significance, but I see no escape from the conclusion that they throw grave doubt on conventional views. Again and again the same question presents itself. If A and B lately emerged from a common form why is that common form so utterly lost that it does not even maintain itself in the region of overlapping? Almost equally difficult is it, in the cases which I have numerated, to apply concrete suggestions based on any factorial scheme. We may see that in Heliconius erato the type with the red mark on the hind wing probably contains a dominant factor, and that where the red mark is absent the metallic colours are exposed; and that similarly the green metallic colour may have another factor which distinguishes it from the blue. In this way we can fairly easily represent the various types of erato on a factorial system as the result of the various possible combinations of two pairs of factors. But there we stop, and we are quite unable to suggest any reason why one area should have the red and the green type while another should have the blue also. So again with Colaptes or the Warblers. By application of a factorial system, admittedly in a somewhat lax fashion, the genetic interrelations of the types can be represented; but how it comes about that each type maintains a high degree of integrity in its own region we can only imagine. Each has in actual fact a stability which the intermediate forms have not, but we cannot yet analyse the nature of that stability. Mendelian conceptions show us how by segregation the integrity of the factors can be in some degree maintained, but not why certain combinations of factors should be exceptionally stable. All that is left us to fall back on is the old unsatisfying suggestions that some combinations may have greater viability than others, that there may be a tendency for like to mate with like, and so forth.
These difficulties acquire more than ordinary force in those cases in which the two fixed types inhabit regions differing in some respect so obvious and definite that we are compelled to regard each type as climatic and as specially adapted to the conditions. When for example an animal has a distinct type never met with except in Arctic or Alpine conditions, and another type proper to the plains and temperate regions, what are the characteristics of the population of intermediate latitudes or at intermediate levels? Some of the examples discussed in the last chapter may be instances of this very nature, but even if they are not, others are forthcoming which certainly are. The evidence of these cases leads to the suspicion that with further knowledge they will be found to consist of two classes, some in which the observer as he passes from the one climate to the other will find the intermediate area actually occupied by a population of intermediate character, and others in which, though we may presume the maintenance of intermediate conditions in the transitional area, there is no definite transitional population. This interrupted or discontinuous distribution seems, so far as I have means of judging, to be by far the more common of the two. I do not doubt that by sufficient search individuals representing every or almost every transitional form can be found, but it is apparently rare that populations corresponding to these several grades can be seen. The question has in few if any cases been studied with precision sufficient to provide a positive answer; but I suspect that real and complete continuity, in the sense thus defined, will only be found where the character of the local populations depends directly on the conditions of life, and shows an immediate response to changes in them apart from that postponed response which we suppose to be achieved by selection. Obviously the character must be one, like size for instance, capable of sensibly complete gradation.
The only example I have met with of the phenomenon of anything like a complete intergradation between local types really distinct in kind is that provided by the butterfly Pararge egeria. It is well known to entomologists that this insect exists in two very different types, a northern one, the "Speckled Wood" of England, in which the spots are a pale whitish yellow, and a southern type having the full fulvous colour that we know as characteristic of megaera, the "Gatekeeper." It appears that Linnaeus gave the name egeria to the southern type,[1] and our own is now called egerides. Broadly speaking, so far as Great Britain, France, and the Spanish Peninsula are concerned, the tawny-coloured egeria occupies Spain and western France up to the latitude of Poitiers and the pale yellow egerides extends from Scotland, where it has a scanty distribution, through southern England, where in suitable localities it is common, and the north of France to Paris.[2] The two types when placed side by side are strikingly different from each other, and are an excellent illustration of what is meant by climatic variation. The insect is not a great traveller and probably scarcely ever wanders far from its home. It should therefore be possible by collecting from north to south to find out how the transition is effected, whether suddenly or gradually. This at various times I have endeavoured to do, but I am still without exact information as to the population in certain critical areas. In addition to the information derived from specimens which I have collected or seen in the collections of others there is a good account of the general distribution in Europe given by the Speyers,[3] who evidently paid more attention to the subject than most lepidopterists have done, and many more recent records. In particular OberthÜr[4] has published many details as to the distribution in western France and I am especially indebted to Mr. H. Rowland-Brown for a long series of notes as to the distribution in France generally, and to Mr. H. E. Page and Dr. T. A. Chapman, Mr. OberthÜr Prof. Arrigoni degli Oddi, Mr. H. Williams and other correspondents, for showing me forms from many localities. The butterfly is attached for the most part to woods of deciduous trees and to country abounding in tall hedges or rough scrub. It is not usually to be found in highly cultivated districts or in very dry regions. Hence there is necessarily some want of continuity in the distribution at the present time and I should think a mile or two of arable land without big hedges would constitute a barrier hardly ever passed. The larva feeds on several coarse grasses, especially Dactylis glomerata. Barrett mentions also Triticum repens. In this country the winter is usually passed in the larval stage, but I have found that in captivity, at least, there is much irregularity. The larvÆ feed whenever the weather is not very cold and may pupate, but if sharp cold comes on when they are pupating or nearly full-grown they often get killed unless protected.
Some writers speak of a difference between the early and later broods, but I have never noticed this, and I do not think that the general tone of the yellow is affected by the seasons (see Tutt, Ent. Rec., IX, 1897, p. 37).[5]
Beginning at the south of Spain the thoroughly fulvous type egeria is common at Gibraltar in the Cork woods, at Granada, and doubtless generally. Lederer is said to have found only this type in Spain (Speyer), and though I have no precise information as to other places in the Peninsula north of Jaen I feel tolerably sure that there is no change from south to north.[6] Immediately north of the Pyrenees we still meet egeria exclusively, and up to Poitiers at least there is no noticeable change. But somewhere between Poitiers and the bottom of the Loire valley at Tours, the genuine southern type comes to an end, and the whole population begins at the Loire to be of an intermediate type, easy to distinguish both from egeria and from egerides. As to the exact condition of the species in the fifty miles separating St. Savin on the Vienne from places on the Loire I have no adequate information. I have only one small sample from there, but it does contain insects both of the southern and intermediate types taken on the same day, in a wood near Preuilly. OberthÜr also states that at Nantes the true southern form exists in company with the northern. From this I infer that the southern form extends up the coast further than it does inland, but I imagine the representative spoken of as northern would be of usual Brittany or intermediate type.
The Vienne river joins the Loire, so the true southern type reaches over into the basin of the Loire. From the Loire (Tours, CormÉry) north to Calvados (Balleroy) only the intermediate is found, so far as I know, and the same type extends over Brittany.[7] In general, however, the woods near Paris have the thoroughly northern type egerides, but at St. Germain-en-Laye and at Etampes (OberthÜr) the population approaches the intermediate type.
On the whole the intermediate type is certainly less homogeneous than either of the extremes, and females with the two central spots either paler or more fulvous than the rest are not uncommon, but I have never taken one on the Loire or in Brittany which I should class with either of the extreme types.
Before speaking of the distribution in other parts of France and in Europe generally I will briefly state the results of my breeding experiments. The work was done many years ago before we had the Mendelian clue, and it is greatly to be hoped that some one will find opportunities of repeating it. Crossing the English and the thoroughly southern type the families produced agree entirely with the intermediates of Brittany and the Loire. Reciprocals are alike. Of F2 I only succeeded in raising very few and of those that I had (about 30) nearly all were intermediate in character, though perhaps rather less uniform than F1. One family alone, containing only 4 specimens, had one egerides, and three fulvous intermediates. As the case stands alone I hesitate whether or not to suppose it due to some mistake. Moreover from F1 crossed back with the respective parental types I had fairly long series, especially from F1 × the southern type, and looking at these families I cannot see any clear evidence of segregation. On the contrary, I think that though there are slight irregularities, they would, taken as a whole, be classed as coming between the intermediate type and the extreme form used as the second parent. This at least is true when the second parent was of the southern type.
On this evidence I have regarded the case as one in which there is no good evidence of segregation and as conforming most nearly with the conventional view of gradual transition in response to climatic influences. Such influence must however be indirect; for I reared five generations of the northern type in England, and these, though they included several abnormal-looking specimens in the last generation and then died out, did not show any noticeable change from the fulvous colour of the wild type. Merrifield[8] also found that heat applied to pupae of the northern type produced no approach to the southern type.
Looking at the facts now in the light of more experience it seems to me just possible that the case may be one in which, as in Nilson-Ehle's Wheats, the dominant differs from the recessive in having two pairs of factors with similar effects. The fulvous type for example may have two or more elements in separate pairs which together produce the full effect, and the intermediate may have one of these. If this were so, some segregation should of course eventually be observable, but the proportion of the various fulvous and fulvous-intermediate individuals would be large, and the reappearance of actual representatives of the northern type might be rare. I admit that this is a somewhat strained interpretation of the facts, and as yet it is not entitled to serious consideration. Nevertheless I am led to form some such expectation partly from the great difficulty in the way of any other, partly from the evidence of the small mixed sample found at Preuilly and partly from the statements given by OberthÜr. There are moreover other features in the general distribution of the species which make it improbable that the dependence on climate can after all be so close. Published lists are unfortunately of little use in deciding which form occurs at a particular place, because, since the name Meone has ceased to be used for the southern form, there is no complete unanimity among authors as to the application of the names egeria and egerides, and unless more particulars are given, either name may be used for either form. Besides this, difficulty arises from the fact that the intermediate type is not generally distinguished at all, and English collectors finding it, may easily record it as the southern type. From Staudinger's note on the distribution, I gather that he, on the contrary, reckoned the intermediate with the northern type, as do the Speyers also. The late Mr. J. W. Tutt was careful to distinguish the three forms and has left several useful records. Easy therefore as it might seem to be to make out the distribution of such a familiar insect in its various modifications, there are serious practical difficulties, and until long series are brought together with this special object in view many obscurities will remain.
With only the series from England, the west of France, and Spain before one it would be easy to regard the successive series of tones as a fair measure of climate; the brighter the colour, the hotter might one expect the locality to be. Such rough correspondence is often to be observed in butterflies and birds. It becomes impossible to take these simple views in the light of more complete knowledge. Beginning with France the fulvous egeria occupies the lower valley of the Rhone, probably from well above Lyon, though I have no exact information respecting the country above Avignon. According to Speyer it also takes the department of LozÈre. The same authority says that Puy-de-DÔme has "egeria," meaning perhaps the intermediate form, with the fulvous form much less commonly. Next comes the curious fact that though the Lower Rhone (Avignon, Tarascon, NÎmes) has the true fulvous form, HyÈres, Cannes, Grasse, Nice, Digne, and Alassio have the intermediate. Savoy has the intermediate (ChambÉry) and even egerides perhaps, though in the same latitude on the west of France there is nothing but the fulvous type. At Chalseul and BesanÇon (Doubs) the ordinary northern type is found. Switzerland generally, I believe, has the northern type, but Staudinger gives egeria for Valais and the intermediate occurs in Vaud.[9] The south side of the Alps has probably colonies of the pale egerides, and of intermediates. Orta, with a very hot summer, has the English type (Tutt, Ent. Rec., XII, 1900, p. 328). Locarno has the intermediate (ibid., XV, 1903, p. 321). North Italy in general and western Piedmont have the intermediate; but further south egeria begins, at what region I do not know. Speyer gives on his own authority the remarkable statement that at Florence both extremes occur, but chiefly intermediates between the two. Mr. R. Verity however kindly informs me that in his experience this is not so, and that neither the real southern type nor the northern occur there. Sardinia, Sicily, Crete all have the southern type. Greece probably has various types. Staudinger (Hor. Ross., VII, 1870, p. 78) says intermediates resembling Nice types common everywhere, but from "Greece" the British Museum has a series that would pass for English specimens; and the same type occurs near Constantinople. The island of Corfu has a pale intermediate, distinct from egerides but approaching it. In Roumania all three forms are recorded from various places: egeria in the Dobrutscha; not quite typical (presumably an intermediate) at Bukharest; intermediate in various mountainous localities as well as in Macedonia and Dalmatia; but egerides in Azuga at about 3,000 feet.[10] Hungary has the true egerides also. (Cf. Caradja, Deut. Ent. Zt., IX, p. 58.) Mathew records the same from Gallipoli (E. M. M., 1881, p. 95). Staudinger does not distinguish the intermediates from the northern, but he gives "egerides" for Armenia and Fergana (Central Asia). As against the mere proximity of a great mountain chain being the influence which keeps the Riviera population intermediate may be mentioned the fact that the northern foothills of the Pyrenees have the pure southern type, and the climate of Cambo must surely be far cooler than that of Nice. The exact locality of the Greek specimens is not given, but there can be no part of Greece which is not much hotter in summer than Brittany, or Calvados, which have the intermediate, not the English type.
In face of these facts it can scarcely be maintained that average temperature is the efficient cause of the particular tone of colour which the butterfly shows in a given region. Nevertheless it is clear that climate counts for much in determining the distribution. It is noticeable that though the pale egerides can be established in a warm climate we never find egeria in cold climates, and even the intermediate is not found in places that have a hard winter. I suspect that the distribution of the broods through the year and the condition of the animal at the onset of hard frost are features which really determine whether a strain can live in a particular place or not. Though the truth of the suggestion cannot be tested by experiments in captivity, which at once introduce disturbances, I incline to the idea that egeria has not got the right periodicity for northern climates. If it could arrange its life so that the population consisted either of young larvae, or perhaps of thoroughly formed pupae[11] at the onset of winter, it might, for any obvious reason to the contrary, be able to live in England. It is irregularly "polyvoltine," as the silk-worm breeders say, and as soon as a little warmth encourages it, a new generation starts into being, which if the frost comes at an untimely moment, is immediately destroyed. Many species are continually throwing off individuals which feed up fast[12] and emerge at once if the temperature permits, and I imagine a species of Satyrid wholly or largely represented by such individuals could scarcely survive in a country which had a hard winter. For such a climate some definite periodicity in the appearance of the broods may well be indispensable. But assuming that egeria is cut off from cold climates for such a reason, there is nothing yet to connect these habits with the fulvous colour, and until breeding can be carried out on a satisfactory scale there is no more to be said.
From time to time records appear of individual specimens more or less fulvous being caught in southern England, especially in the New Forest.[13] It would be interesting to know what offspring such individuals might produce. From the evidence now given some notion both of the strength and the weakness of the case considered as one of continuous climatic variation can be formed. I know no other equally satisfactory. Whether or not definite mixture of the intermediates with either of the extremes will be proved to occur, the case differs materially from those considered in the last chapter in the fact that at all events there is no general overlapping of forms. In a species so little given to wandering, overlapping could indeed scarcely be expected to occur. It is this circumstance which makes the species preeminently suitable as a subject for the study of climatic influences, and I trust that entomologists with the right opportunities may be disposed to explore the facts further.
Just as many species, like egeria, have varieties which can be regarded as adapted to northern and southern regions, so there are also several which have lowland and Alpine forms quite distinct from each other. Every such case presents an example of the problem we have been considering. As the collector passes from the plains to the Alpine region, how will he find the transition from one form to the other effected? Does the lowland form give place to the Alpine form suddenly, with a region in which the two are mixed, or will he find a zone inhabited by an intermediate population? I have spent a good deal of time examining the facts in the case of Pieris napi and its Alpine female variety bryoniae, and though there are many complications which still have to be cleared up, no doubt is possible as to the main lines of the answer. If in any valley in the Alps inhabited by both napi and bryoniae the collector catches every specimen he can, beginning at the bottom and working up to 7,000 feet, he will at first get nothing but napi. At about 2,500 feet, he may catch an occasional bryoniae flying with the napi. After 3,000 feet napi usually ceases, and only bryoniae are found. As an exception a colony of napi may be met with at much greater heights. I once found them in numbers at about 6,000 feet.[14] Not only were they free from any trace of modification in the direction of bryoniae, but they were of the thoroughly southern type of napi, being a late brood of that large and very pale kind (meridionalis) almost destitute both of dark veining above and of green veining below, which are common on the shores of Lago Maggiore and in other hot southern localities. Not far off at the same level were typical bryoniae in fair abundance. Occasionally an intermediate may be met with. I have taken a few, for example, at Macugnaga and at Fobello. These, however, in my experience are rarities in the Alps. Fleck[15] gives notes on the distribution in Roumania which shows the same state of things. The lowland form is not transformed though found at great heights, and at Azuga (nearly 3,000 feet) bryoniae occurs with only occasional "flavescens," viz., intermediates of the second brood.
If this were all the evidence we should be satisfied that the lowland and Alpine types keep practically distinct, overlapping occasionally, but rarely interbreeding. The problem would remain, how is the distinctness of the two types maintained in the region of overlapping? Nowadays, I suppose, we should incline to answer this question by reference to segregation, and perhaps by an appeal to selective mating. The suggestion that segregation does take place is certainly true to some extent. There are, however, difficulties in the way, and the whole subject is one of great complexity. My own experiments were made in pre-Mendelian times and were not arranged with the simplicity which we now know to be essential. The results are neither extensive enough nor clear enough to settle the many collateral questions which have to be considered, and the work ought to be done again. Nevertheless, some notes of the observations may have a suggestive value.
When I began, I did not sufficiently appreciate that the "napi" group, omitting the North American forms, and the Asiatic representatives, has at least three chief types in western Europe. The differences we have to deal with are manifested by the females only, so in this account particulars as to the males are omitted for the most part. These are (1) our own British napi; (2) the form found in the south, from the Loire downwards, and in the Italian Alps, which I think may be spoken of as meridionalis; (3) bryoniae, which is a form clearly recognizable in the female only, and is found only in the arctic regions and in the Alps above 2,500 feet. The first two have several broods, two, three, or more, according to opportunity, and the first brood is different from the later ones. In napi the markings on the upper surface are a dark grey but in meridionalis they are a pale silvery grey and much less extensive. In the later broods of napi there is much less general irroration of the veins, and the spots stand out as more defined and blacker. These differences vary greatly in degree of emphasis. In meridionalis the later broods are entirely different from the first. Instead of having silvery markings they have the ground colour quite white, with the spots large and a full black. On the under side of the hind wings the usual green veins are almost absent, and I have seen individuals which could scarcely be distinguished from rapae. To these later broods the term napaeae is sometimes applied, but I here use meridionalis for the southern race in general as applicable to all broods.
The female bryoniae is totally unlike the others. The ground colour is a full yellow, and each nervure is thickly irrorated with a brown pigment often spreading so far as to hide the ground almost entirely in the fore-wings. The males corresponding with these females are not certainly distinguishable from those of our own napi. Both sexes have the green veining of the underside of the hind wing fully developed, rather more than is usual in the lowland races, but this is not really diagnostic of the variety. The first serious difficulty arises in regard to the second brood of bryoniae. It is stated that there is only one brood,[16] but I feel fairly sure that a second brood is sometimes produced, and that the females with a yellow ground and diminished irroration of the veins, not very uncommon in the Italian Alps in July to August, are generally representatives of it. Such insects would of course be classed with bryoniae in collections.
My experiments began with eggs of true bryoniae females caught at about 2,500 feet early in July. These emerged in August-September as intermediates with yellow ground and about half as much black on the upper surface as bryoniae. They are exactly like the intermediates usually found in nature and in the light of later experience I regard them as natural F1 forms, and I think the mothers had been fertilised by napi males, though I admit that in view of the rarity of natural intermediates there is a difficulty in this suggestion. Three of these females were mated with males raised from thorough meridionalis females, and three families were produced. Two of them showed distinct evidence of segregation, some being yellow and some white with various intergrades, some being no blacker than meridionalis and some ranging up to a dark intermediate type. Part emerged in the same autumn; and part overwintered, emerging as the spring meridionalis or as the peculiar type which I afterwards learnt to know as the spring F1 form. The distinctions were fairly sharp between the several forms. But the offspring of the third female gave a series practically continuous from meridionalis to the F1 type. The work of subsequent years gave results similarly irregular which could only be described adequately at great length. The outcome may however be summed up in the statement that there is evidence that both the yellow ground and the dark veining are due to factors, but that there are several of these and that imperfect segregation is not uncommon, producing various reduction-stages. The yellow ground may be due to one factor, and the several shades may be the result of irregularities in dominance, but the black markings when fully developed cannot I think be the result of less than three factors, one for the basal darkening, one for general irroration, and one for the margins. Probably also the enlargement of the spots is produced by a fourth factor.
There was not, in my experience any great difficulty in getting the various forms to pair in captivity. Some attempts were made to see whether individuals of either type selected mates of their own type in preference to those of the other, but the results were inconclusive. There were some indications of such a preference; though, from the impossibility of judging how much of this may be due to other circumstances, I could not come to a positive conclusion on the rather meagre evidence.
Recently Schima[17] has given a careful and detailed account of all the forms found in Lower Austria which he enumerates under 14 distinct varietal names. He gives full references to previous accounts, especially to the beautiful plates lately published by Roger Verity.[18] Examination of these and of my own specimens strongly suggests that the several forms are due to the recombination of the factors I have named. Among those which I have bred are representatives of most if not all the types enumerated by Schima in addition to other curious forms. For example I have bryoniae markings on a ground practically white; the dark veins with spots almost obsolete; meridionalis on a yellow ground; the intermediate amount of black on a white ground, etc. The last-named may occur wild and I have one from Macugnaga as well as one given me by Mr. F. Gayner from Lulea (Lapmark).
To obtain really exact knowledge of the number of factors and their properties it would be necessary to repeat the work. After the beginning, I made a mistake in using British napi instead of meridionalis and the results were much confused thereby. The contrast between meridionalis and the various dark forms is much greater and classification of the types would have been therefore easier. The British form is presumably meridionalis plus the factor for the basal pigmentation. The problem is greatly complicated by the differentiation of the seasonal forms. The first point to be determined is whether bryoniae is capable of producing a second brood when it is thoroughly pure-bred, and whether such a second brood is, as I suspect, normally intermediate in character.
In the Alps generally there is no definitely intermediate population; nor I believe, is any such population met with in the north where the arctic bryoniae meets napi, but as to this I have no precise information. One curious fact, however, must be mentioned, namely that there is a population that can probably be so described with fairness established at MÖdling near Vienna. This is not in any sense an Alpine locality, and does not, as I am told, differ in any obvious way from the other suburbs of Vienna. Dr. H. Przibram was so good as to send me a set taken at this place, representing a second brood, and they were decidedly heterogeneous, ranging from an intermediate form such as bryoniae fertilised by napi usually produces, to a light yellowish second-brood type with little dark pigment. There are also two actual bryoniae. Whether true napi also occur there I do not know, but I have no doubt they do. It would be well worth while to investigate the MÖdling population statistically, and to breed from the intermediates which might not impossibly prove to be heterozygotes. There are also records of such intermediates being occasionally found in some parts of Ireland, in the north of Scotland, and in south Wales,[19] but I do not know of any regular colony of these forms. We can scarcely avoid the inference that one or more of the factors which make up bryoniae may be carried by these intermediates. It is not clear why their interbreeding does not produce actual bryoniae occasionally. If this occurred, the probability is that the fact would be known to collectors, at least in the British localities. The absence of true bryoniae must, I think, be taken to mean that some essential factor is absent from these intermediates.
To sum up the evidence, the facts that are clear may be thus enumerated:
1. Napi and bryoniae, or in the Italian Alps, napaeae and bryoniae frequently meet each other.
2. They cross without difficulty, producing fertile offspring.
3. But in the levels at which they overlap there is no intermediate population, and only occasional intermediate individuals.
4. In certain parts of the distribution of napi similar intermediates sometimes occur, and at one place (MÖdling) they are so frequent as apparently to constitute a colony.
5. As to the genetic relations of the two forms there is no complete certainty. Indications of segregation have been observed in some cases, but there are several factors concerned and they are liable to some disintegration.
Another form in which I tried to investigate the same problem is Coenonympha arcania, which has one Alpine form known as darwiniana, and another, satyrion. In calling satyrion a form of arcania I follow Staudinger and other authorities, but I have never been quite satisfied that it should be so regarded. The differences between arcania and darwiniana are essentially differences of degree; C. arcania occurs in places where there is cover, and reaches up the valleys usually as high as the mixed woods of deciduous trees, which is about 2,500 feet. The variety darwiniana, on the contrary, is an insect of treeless hillsides, and I regard it as a dwarf and possibly a stunted form. It would not greatly surprise me to find that with the application of good conditions arcania could be raised from darwiniana eggs, or that if arcania larvae were starved they might give rise to darwiniana butterflies. I have been unsuccessful in trying to rear the species, having lost the larvae by disease. Usually one does not catch arcania and darwiniana on the same ground, and as Festuca ovina—a typically hill-side grass—is a common food-plant of darwiniana there can be little doubt that arcania feeds on some other grass, probably woodland species. Colonies of arcania of varying size and brightness are commonly found, and though a sample of arcania, finely grown, from a warm Italian wood, presents a striking contrast with darwiniana from an Alpine pasture, one certainly may get samples which fill all the gradations. Generally the sample from a given locality is fairly homogeneous.
Of satyrion I have little personal experience. I only twice found it, namely at Zinal, and at Hallstatt in Austria, but it occurs at Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and I understand that it is the typical Alpine form in the Engadine. With its darkened colour and reduced size it might well be expected to be a still further stunted form of darwiniana. Yet I have never found the one succeed to the other at the higher levels. If darwiniana appears when Alpine conditions are reached in a valley it will be met with up to the highest level at which such butterflies live. Tutt was of opinion that satyrion is a distinct species.[20] I once, at the top of the Vorderrheinthal caught a sample of darwiniana a few of which (males) were so dark and had the eye spots so poorly developed that they looked like transitions to satyrion. Otherwise I never found any such transitional forms and they are certainly exceptional. There is further a record[21] of satyrion having been taken flying with arcania. This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has similar specimens from Caud and from St. Anton (Arlberg). The females, however, both of mine and of Mr. Page's samples are a pale brown, quite unlike the females both of arcania and of the dark Zinal satyrion. The difficulty thus raised has not I think yet been considered by the authorities, and it is possible that the Alpine forms of arcania are in reality three, not two.
The evidence taken together suggests, I think, that darwiniana is related to arcania much as so many of the Alpine varieties of plants are to the well-developed individuals of the lower levels. I do not anticipate that factorial differences will be found in these insects, and it is by no means impossible that the distinctions between them are the direct consequences of altered conditions. The relations of arcania to satyrion are more doubtful, and in that case a factorial difference may at least be suspected.
The species of the genus Setina have Alpine forms which agree in possessing a characteristic extension of the black pigment to form radiating junctions between the spots on the wings. Speyer, who discussed the interrelations of these forms in detail,[22] lays stress on the absence of genuine transitional forms between aurita and the variety ramosa. Both are mountain insects but ramosa extends to levels higher than that at which aurita ceases, which is about 4,000 feet. The two forms are often found flying together. Speyer says that his brother searched diligently for transitional forms at the level of overlapping, but found none, so that at least they may be regarded as rare. The variety ramosa is not infrequent at much lower levels (e. g., Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high as the permanent snows. In the British Museum collection, however, I have seen several that I should regard as transitional. Speyer perhaps would have classed as ramosa all in which the spots of the central field were united, and it is by no means unlikely that breeding would prove such individuals to be heterozygous.[23]
There can scarcely be a doubt that the distinction between aurita and ramosa is factorial, the radiate ramosa probably having the factor for striping. In support of this view may be mentioned the observation of Boisduval,[24] respecting a gynandromorphous individual, which was aurita male on one side, and ramosa female on the other. Speyer makes another excellent comment. He points out that the simple notion that the radiation is a mere extension of pigmentation consequent on the climate of the higher levels, will not fit the facts very easily, because the size of the spots varies greatly in aurita itself at any level, and lowland specimens may actually have more black confined to the spots alone than some ramosa possess on spots and lines combined.[25]
The two Salamanders, S. maculosa and its Alpine form atra, might not improbably furnish evidence bearing on the same problem. The two are of course very distinct, not merely in colour (maculosa being spotted with yellow or orange while atra is entirely black) but also in the mode of reproduction, a feature to which reference will be made in the next chapter. I cannot, however, find any evidence as to the overlapping of the two forms. S. atra occurs from about 3,000 feet or somewhat less, and reaches great elevations in the Eastern Alps, but I do not know if the two forms ever occur in the same localities. Leydig,[26] Boulenger,[27] and most modern authorities regard the two types as distinct species, but they are in any case closely allied, and it would be of interest to have exact knowledge of their geographical delimitations.
The reader who has considered the cases adduced will appreciate the difficulties which must be faced in any attempt to account for the facts in a rational way. As always in a problem of Evolution, two separate questions have to be answered. First how did the form under consideration come into existence, and secondly, how did it succeed in maintaining itself so as to become a race? The evidence from the local forms, though very far from giving complete answers to either of these questions definitely refutes the popular notion that a new race comes into existence by transformation of an older race. If a gradual mass-transformation of this kind took place we should certainly expect that when two types, nearly allied and capable of interbreeding, overlap each other in their geographical distribution, a normally intermediate population would exist. If each type can maintain itself, and if each came into existence by gradual transformation, then there must have been an intermediate capable of existing and maintaining itself as a population; and if this had ever been, surely in the region of overlapping, that intermediate population should continue. Especially should such a population be found when the two extreme types are adaptational forms and the region of overlap is a region of intermediate conditions. But of the examples we have examined there is only one, that of Pararge egeria and egerides, which can at all be so interpreted, and even in that case it is not impossible that more minute observation would reveal discontinuity between the extremes and the admittedly normal intermediate population. Granting provisionally however that this example, as it stands, is consistent with the conventional theory of evolution, I know not where we should look for another case equally good. When the distinctions are produced by direct influence of conditions operating during the lifetime of the individuals, examples of intermediate populations occupying the areas of intermediate conditions can no doubt be produced. Many turf-like Alpine plants, for instance, if protected from exposure and properly nourished can grow as large as those of the same species found in the valleys, and in the case of such quantitative effects, intermediate conditions can doubtless produce intermediate characters.
Even these examples however are not very abundant, and often the intermediate locality has not a form intermediate between those of the two extreme localities, but some third form distinct from either. This is the case for instance in the fauna of brackish waters. We are taught to believe that the fresh water fauna was evolved from the marine fauna, which it well may have been; but as students of Crustacea and Mollusca know familiarly, the brackish water forms are not as a rule intermediates between fresh water species and sea species, but more usually they are special forms belonging to the brackish waters, with the peculiar property that they can tolerate a great range of conditions, and live without ostensible variation in waters of most various compositions and densities, which very few marine or fresh water species are able to do.
Sometimes the distinction between local races, as in Rhamphocoelus passerinii and icteronotus may be regarded with confidence as due to one simple Mendelian factor possessed by one race and absent from the other, but I think, more often, as in Colaptes or in the varieties of Pieris napi, the existence of several distinct factors is to be inferred. As we have seen, the races of Colaptes show almost beyond doubt that in different areas at least three distinct factorial combinations can be perpetuated as races.
In the distribution of variability we find, I think, some hint as to the steps by which the phenomena under consideration have come to their present stage, and I am disposed to regard the facts so well attested in the case of our own melanic moths as a true indication of the process. Following this indication we should regard the change in the character of a population as beginning sporadically, by the appearance of varying individuals, possibly only one varying individual, in, it may be, one place only. As to why a variety should increase in numbers we have nothing but mere speculation to offer, and for the present we must simply recognise the fact that it may. That such survival and replacement may reasonably be taken as an indication that the replacing race has some superior power of holding its own I am quite disposed to admit. Nevertheless it seems in the highest degree unlikely that the outward and perceptible character or characters which we recognise as differentiating the race should be the actual features which contribute effectively to that result.
In discussions of geographical distribution in relation to problems of origin it is generally said that very nearly allied species usually occupy distinct areas, while other competent observers state the exact contrary. Lately, for example, Dr. R. G. Leavitt[28] has published an important collection of evidence upholding the latter proposition, taken chiefly from the botanical side, showing how in numerous genera two or more closely allied species coexist, frequently without intermediates, in the same localities, and may even be thus found in company throughout their distribution. The difference of opinion evidently arises from a confusion as to the sense in which the term "species" is understood and applied. Leavitt, for example, is avowedly following Jordan and, among moderns, Sargent, in applying a close analysis, and denoting as species all forms which are distinct and breed true. Against this use of the term I know no valid objection[29] but it must be obvious that if others follow a different practice confusion may result when observations are summarised in general statements. We will consider this subject again in another place, but here it may be sufficient to say that there can scarcely now be a doubt that numbers of these associated species, such as Jordan discriminated, represent various combinations of the presence and absence of Mendelian factors. This does not in any way weaken the argument which Leavitt founds upon the facts, namely, that the observed distribution of these forms is consistent with the supposition of an evolution largely discontinuous.
On the other hand, those who have come to the opinion that nearly allied species generally occupy distinct ground are presumably more impressed by the characters differentiating the geographically distinct or adaptational races, seeing that genuine intermediates between them are less commonly found. Those geographical races may no doubt contain various differentiated forms; but when all live together, occasional intermediates are usually to be found even in the case of characters habitually segregating. These segregating forms Jordan would certainly have determined as species, and it must be conceded that no physiological definition has yet been drawn which consistently excludes them.
