LECTURE XXXII
INFLUENCE OF ISOLATION ON THE FORMATION
OF SPECIES
Introduction—Isolated regions are rich in endemic species—Is isolation a condition in the origin of species?—Moriz Wagner, Romanes—'Amiktic' local forms, the butterflies of Sardinia, of the Alps, and of the Arctic zone—Periods of constancy and periods of variation in species—Amixia furthered by germinal selection—The thrushes of the Galapagos Islands—The intervention of sexual selection—Humming-birds—Central American thrushes—Weaver-birds of South Africa—Papilionidæ of the Malay Archipelago—Natural selection and isolation—Snails of the Sandwich Islands—Influences of variational periods—Comparison with the edible snail and with the snail fauna of Ireland and England—Changed conditions do not always give rise to variation—Summary.
In an earlier lecture I endeavoured to show, by means of Darwinian arguments and examples, how important for every species, in relation to its transmutation, is the companionship of the other species which live with it in the same area. We saw that the 'conditions of life' operated as a determining factor in the composition of an animal and plant association quite as momentously as any climatic conditions whatsoever, and, indeed, Darwin rated the influences of vital association even more highly, and attributed to them an even greater power of evoking adaptation than he granted to the physical conditions of life.
We are, therefore, prepared to recognize that even the transference of a species to a different fauna or flora may cause it to vary, and this occurs when the species gradually extends the area of its distribution, so that it penetrates into regions which contain a materially different association of forms of life. But these migrations are not necessarily only gradual, that is, due to the slow extension of the original area of distribution in the course of generations as the species increases in numbers; they may also occur suddenly, when isolated individuals or small companies of a species transcend in some unusual manner the natural boundaries of the old area, and reach some distant new region in which they are able to thrive.
Species-colonies of this kind may be due to the agency of man, who has spread many of his domesticated animals and plants widely over the earth, but who has also intentionally or unintentionally forced many wild animals and plants from their original area to distant parts of the earth, as, for instance, when the English humble-bees were imported into New Zealand with a view to securing the fertilization of the clover; but such colonies also occur in thousands of cases independently of man's agency, and the means by which they are brought about are very diverse. Little singing-birds are sometimes driven astray by storms, and carried far away across the sea, to find, if fortune favours them, a new home on some remote oceanic island; fresh-water snails, which have just emerged from the egg, creep on to the broad, webbed feet or among the plumage of a wild duck or some other migratory bird, and are carried by it far over land and sea, and finally deposited in a distant marsh or lake. This must happen not infrequently, as is evidenced by the wide distribution of our Central European fresh-water snails towards the north and south. But terrestrial snails can also, though more rarely, be borne in passive migration far beyond limits which are apparently impassable, as is evidenced by the presence of land-snails on remote oceanic islands.
The Sandwich Islands are more than 4,000 kilometres from the continent of America; they originated as volcanoes in the midst of the Pacific Ocean; and yet they possess a rich fauna of terrestrial snails, the beginnings of which can only have reached them by the chance importation of individual snails carried by strayed land-birds. Charles Darwin was the first who attempted to investigate the problem of the colonization of oceanic islands by animal inhabitants, and the chapter in The Origin of Species which deals with the geographical distribution of animals and plants still forms the basis of all the investigations directed towards this point. We learn from these that many land-animals, of which one would not expect it on a priori grounds, may be carried away by chance over the ocean, either, as in the case of butterflies and other flying insects, and of birds and bats, by being driven out of their course by the wind, or by being concealed—either as eggs or as fully-formed animals—in the clefts of driftwood, where they can resist for a considerable time the usually destructive influence of salt water. Thus eggs of some of the lowest Crustacea (Daphnidæ), which are contained in large numbers in the mud of fresh water, may be transported with some of the mud on the feet of birds, and this may happen also to encysted infusorians and other unicellulars, and to the much more highly organized Rotifers as well. In all these cases, and in many others, it may happen occasionally that single individuals, or a few at a time, may be carried far afield, and may reach regions from which their fellows of the same species are entirely excluded. If they thrive there they may establish colonies which will gradually spread all over the isolated area as far as it affords favourable conditions of life.
But oceanic islands are not the only cases of isolated regions; mountains and mountain-ranges which rise in the midst of a plain also form isolation-areas for mountain-dwelling plants or animals which have not much power of migrating. In the same way marine animals may be completely isolated from each other by land-barriers, as the inhabitants of the Red Sea, for instance, are from those of the Mediterranean, as has been clearly expounded by Darwin. The idea of an isolated region is always a relative one, and the region which seems absolutely insular for a terrestrial snail is not so at all for a strong-flying sea-bird. There is no such thing as absolute isolation of any existing colony, for otherwise the colony could never have reached the region; but the degree of isolation may be absolute as far as the time of our observation is concerned, if the transportation of the species concerned occurs so rarely that we cannot observe it in centuries, or perhaps in thousands of years, or if the extension of its range could only take place through climatic or geological changes, such as a subsidence of land-barriers between previously separated portions of the sea, or, in the case of land animals such as snails, the elevation of the sea-floor and the filling up of arms of the sea which had separated two land-areas. But even the transportation of a species by the accidental means already indicated will occur so rarely, if the isolated insular region is very distant, that the isolation of a colony by such a chance may be regarded as almost absolute as far as the members of the same species in the original habitat are concerned.
If we examine one of these insular regions with reference to the animal inhabitants which live upon it in isolation, we are confronted with the surprising fact that it harbours numerous so-called endemic species, that is to say, species which occur nowhere else upon the earth, and that these species are the more numerous the further the island is removed from the nearest area of related species. It looks at first sight quite as if isolation alone were a direct cause of the transformation of species.
The facts which seem to point in this direction are so numerous that I can only select a few of them. The Sandwich Islands, to which we have already referred, possess eighteen endemic land-birds, and no fewer than 400 endemic terrestrial snails, all belonging to the family group of Achatinellinæ, which occurs there alone.
The Galapagos Islands lie 1,000 kilometres distant from the coast of South America, and they too harbour twenty-one endemic species of land-birds, among them a duck, a buzzard, and about a dozen different but nearly related mocking-birds, each of which is found in only one or two of the fifteen islands. The group of islands also possesses peculiar reptiles, and they take their name from the gigantic land-tortoises, sometimes 400 kilogrammes in weight, which in Tertiary times inhabited also the continent of South America, but are now found in the Galapagos Islands only. The islands also possess endemic lizards of the genus Tropidurus, and although the lizards can no more have been transported across the ocean than the tortoises, but corroborate the conclusion drawn from geological data, that the islands were still connected with the mainland in Tertiary times, the occurrence of a particular species of Tropidurus upon almost every one of the fifteen islands testifies anew to the mysterious influence of isolation, for most of these islands are quite isolated regions for the different species of lizard, even more than for the mocking-birds, which have also split up into a series of species.
We are thus led to the hypothesis, which was first introduced into the Evolution Theory by Darwin, that the prevention of constant crossing of an isolated colony with the others of the same species from the original habitat favours the origin of new endemic species, and his conclusion is confirmed when we learn that islands like the Galapagos group possess twenty-one endemic land-birds, but only two endemic sea-birds out of eleven, for the latter traverse great stretches of sea, and crossings with others of the same species on the neighbouring continental coasts will often take place. The Bermuda Islands also afford a proof that the development of endemic species is prevented by regular crossing with other members of the species from the original habitat, for although they are 1,200 kilometres distant from the continent of North America—that is, further than the Galapagos Islands from South America—they possess no endemic species of bird, and we may undoubtedly associate this with the fact that the migratory birds from the continent visit the Bermudas every year.
Madeira also confirms our conclusion, for only one of the ninety-nine species of bird occurring there can be regarded as endemic, and it has often been observed that birds from the neighbouring African mainland (only 240 kilometres distant) are driven across to Madeira. Terrestrial snails, on the other hand, will seldom be carried to Madeira by birds, and accordingly we find there an extraordinary number of endemic terrestrial snails, namely, 109 species.
Although these and similar facts indicate strongly that isolation favours the evolution of new species, it would be erroneous to imagine that every isolation of a species-colony conditions its transmutation to a new species, or, as has been maintained, first by Moritz Wagner, and later by Gulick and by Dixon, that isolation is a necessary preliminary to the variation of species—that not selection but isolation alone renders the transmutation of a species possible, and thus admits of its segregation into several different groups of forms. Romanes went so far as to regard the natural selection of Darwin and Wallace as a sub-species of isolation, and isolation in its diverse forms he regarded as the sole factor in the formation of species. He assumed that it was only by the segregation of individuals which did not vary that the constant reversion to the ancestral species could be prevented, and he regarded the process of selection as essentially resulting in the 'isolation' of the fittest through the elimination of the less-fit. The idea is correct in so far, that selection undoubtedly aids the favourable variation to conquest over the old forms, precisely because the latter, being less favourably placed in the struggle for existence, are gradually more completely overcome and weeded out, so that a constant mingling of the new forms with the old is prevented, just as it is by isolation of locality. Obviously the new and fitter forms could not become dominant, could not even become permanent, if they were always being mingled again with the old. But whether it serves any useful purpose to bring this under the category of 'isolation,' and to say that mingling with the ancestral form during transmutation is prevented by natural selection, in that favourably varying individuals are isolated by their superiority from the inferior ones, that is, the non-varying individuals which are doomed to elimination, is somewhat doubtful. For my part, I should prefer to retain the original meaning of the word, and to call 'isolation' the separation of a species-colony by spatial barriers.
Whether this factor by itself prevents the mingling with the ancestral form as effectually as selection does, and whether isolation alone and by itself can lead to the evolution of new forms, or perhaps must lead to them, must now be investigated.
I look at this question from exactly the same point of view as I did nearly thirty years ago, when in a short paper[27] I endeavoured to show that, under favourable circumstances, an individual variation of a species may become the origin of a local variety if it finds itself in an isolated region. Suppose an island had no diurnal butterflies, until one day a fertilized female of a species from the continent was driven thither, found suitable conditions of life, laid its eggs, and became the founder of a colony; the prevention of constant crossing between this colony and the ancestral continental species would not in itself be any reason why the colony should develop into a variety. But suppose that the foundress of the colony diverged in some unimportant detail of colouring, such as may at any time arise through germinal selection, from the ancestral species; then this variation would be transmitted to a portion of her progeny, and there would thus be a possibility that a variety should establish itself upon the island which would be the mean of the characters of the surviving progeny. The greater the divergence was in the first progeny of the mother-colonist, and the stronger this variational tendency was, the greater also would be the chance that it would be transmitted further and become a characteristic aberration from the marking of the original species. I then designated this effect of isolation as due to amixia, that is, to the mere prevention of crossing with the members of the same species in the original habitat.
[27] Ueber den Einfluss der Isolirung auf die Artbildung, Leipzig, 1872.
We have examples of this from the Mediterranean islands, Sardinia and Corsica, which possess in common nine endemic varieties of butterflies, most of which diverge from the species of the continent in a quite inconsiderable degree, though quite definitely and constantly. Thus there flies in these islands a variety (Vanessa ichnusa) of our common little Vanessa urticæ in which the two black spots on the anterior wing exhibited by the original species are wanting. The large tortoise-shell (Vanessa polychloros) also occurs there, but it has not varied and still exhibits the black spots. Our little indigenous butterfly (Pararga megæra), which is abundant on warm, stony slopes, quarries, and roads, flies about in Sardinia, but as a variety (tigelius), which is distinguished from the original species by the absence of a black curved line on the posterior wings.
That of two nearly related and similarly marked species, like the large and small tortoise-shell, one should remain unvaried, while the other has become a variety, shows us that amixia alone does not necessarily lead to the evolution of varieties in every case. It might of course be objected that one species may have migrated to the islands at a much earlier period than the other, and that it might be a direct effect of the climate which found expression in this way. But we have other similar cases in which one of two species has varied in an isolated region, while the other has not, and in regard to which we can prove definitely that both were isolated at the same time.
An instance of this kind is to be found in Arctic and Alpine Lepidoptera, which inhabited the plains of Europe during the Glacial period, and subsequently, when the climate became milder again, migrated some to the north into countries within the Arctic zone, and some to the south to the Alps to escape in their heights from the increasing warmth. There are many diurnal Lepidoptera which now belong to both regions, and of these some have remained exactly alike, so that the Arctic form cannot be distinguished from the Alpine form; others show slight differences, so that we can distinguish an Arctic and an Alpine variety. To the former category belong, for instance, Lycæna donzelii and Lycæna pheretes, Argynnis pales, Erebia manto, and others; to the second category belong, for instance, Lycæna orbitulus, Prun., Lycæna optilete, Argynnis thore, and some species of the genus Erebia.
This cannot be an instance of the direct effect of general climatic influences, for in that case all the nearly related species of a genus would have varied or not varied; nor have we to do with adaptations, for the differences in marking are seen on the upper surfaces of the wing, which do not exhibit protective colouring, at least in these Lepidoptera. It can only have been the prevention of crossing that has fixed the existing variational tendencies in the isolated colonies—variations which would have been swamped and obliterated if there had been constant crossing with all the rest of the members of the species.
But there is another factor to be considered. Those Alpine Lepidoptera, for instance, which have not remained exactly the same in the far north, have formed local varieties in the rest of the area of their distribution also, while species which have remained quite alike in isolated regions, such as the Alps and the north, exhibit no aberrations in other isolated regions, such as the Pyrenees, in Labrador, or in the Altai. Thus one species must have had a tendency in the Glacial period to form local varieties, and the other had not; and I have already attempted to explain this on the hypothesis that the former at the time of their migration and segregation into different colonies were at a period of dominant variability, the latter at a period of relatively great constancy. Leaving aside the question of the causes of this phenomenon, we may take it as certain that there are very variable and very constant species, and it is obvious that colonies which are founded by a very variable species can hardly ever remain exactly identical with the ancestral species; and that several of them will turn out differently, even granting that the conditions of life be exactly the same, for no colony will contain all the variants of the species in the same proportion, but at most only a few of them, and the result of mingling these must ultimately result in the development of a somewhat different constant form in each colonial area.
If we were to try to imitate this 'amixia' artificially we should only require to take at random from the streets of a large town a number of pregnant bitches, and place each of them upon an island not previously inhabited by dogs, and then a different breed of dog would arise upon each of these islands, even if the conditions of life were exactly similar. But if, instead of these variable bitches, the females of a Russian wolf were placed on the islands, the developing wolf-colonies would differ as little from the ancestral species as the various Russian wolves do from one another—similar climate and similar conditions of life being presupposed.
There is thus an evolution of varieties due to amixia alone, and we shall not depreciate the significance of this if we consider that individual variations are the outcome of the fluctuations in the equilibrium of the determinant system of the germ-plasm, to which it is always more or less subject, and that variations of the germ-plasm, whether towards plus or minus, bear within themselves the tendency to go on increasing in the direction in which they have begun, and to become definite variational tendencies. In isolated regions such variational tendencies must continue undisturbed for a long period, because they run less risk of being suppressed by mingling with markedly divergent germ-plasms.
The probability that variational tendencies set up in some ids of the germ-plasm by germinal selection will persist and increase is obviously greater the more the germ-plasms combining in amphimixis resemble each other. For instance, let us call the varying determinants Dv, and assume as a favourable case that these are represented in three-fourths of all the ids in the fertilized eggs of a butterfly-female which has been driven astray on to an island, that is, that they are present in twelve out of sixteen ids; then of 100 offspring of the first generation it is possible that seventy-five or more will contain the determinants Dv, some of them in a smaller number of ids, some in a great number than the mother, according as the reducing division has turned out. If the pairing of the second generation be favourable—and this again is purely a matter of chance—a third generation must arise which would contain the variants Dv throughout, and thus the fixation of this particular variation on this particular island would be begun. In other words, the possibility would arise, that, if individuals with a majority of Dv ids predominated, they would gradually come to be the only ones, since by continual crossing with the minority which possessed only the determinants D, they would mingle the varied ids with those of the descendants of these last, till ultimately germ-plasm with only the old ids would no longer occur.
In following out this process it is not necessary to assume that the first immigrant possessed the variation visibly; if determinants varying in a particular direction occurred in the majority of its ids, these would, as a consequence of persistent germinal selection, go on varying gradually until the externally visible variation appeared. This would not have appeared at all if the animal concerned had remained in the original habitat of its species, for there it would have been surrounded by normal germ-plasms, and its direct descendants, even if they had been as favourably situated for the origin of variations as we have assumed, would not have reproduced only among themselves, and therefore even in the next generation the number of Dv ids would have diminished.
Obviously it is to a certain extent a matter of chance whether in the isolated descendants the variation or the normal form remains the victor, for it depends on the number of Dv ids originally present in the fertilized eggs, then on the chances of reducing divisions, and finally on the chance which brings together for pairing individuals in which the similarly varied ids preponderate. The probability of the conquest of the variation will depend in the main on the strength of the majority of the varied ids in the fertilized eggs of the parents; if this be an overwhelming majority, then the chances of favourable reducing divisions and pairings will also be great. The origin of a pure amixia variety will thus depend upon the fact that the same variational tendency Dv was present in a large number of the ids of the ancestral germ-plasm. We need not wonder therefore that of the numerous diurnal butterflies of Corsica and Sardinia only eight have developed into endemic, probably 'amiktic,' varieties.
But since we know that so many species in oceanic islands and other isolated regions are endemic or autochthonous, i.e. of local origin, there must obviously be some other factor in their evolution in addition to the mere prevention of crossing with unvaried individuals of the same species. The variational tendencies which have arisen in the germ-plasm through germinal selection may—as we have already seen—gain the ascendancy in various ways; first, by being favoured by the climatic influences, then by being taken under the protection of personal selection, whether in the form of natural or of sexual selection.
As the inhabitants of insular areas are not infrequently subject to special climatic conditions, we may assume at the outset that many of the 'endemic' species are climatic varieties, but in many cases this explanation is insufficient. For instance, special local forms of mocking-bird live on several of the Galapagos Islands, but this cannot depend upon differences of climate, for the islands are only a few kilometres apart, and resemble one another as regards the conditions of life which they present. But as the differences between these local forms show themselves especially in the male sex, as colour variations of certain parts of the plumage, we must take account of sexual selection, which, though with its basis in germinal selection, has in many islands followed a path of its own. Sexual selection operates especially in the case of sporadically occurring characters which are in any way conspicuous. But it is just such variations as these that are called into existence by germinal selection, whenever it is allowed to continue its course undisturbed through a long series of generations. Characters of this kind, such, for instance, as feathers of abnormal structure or colour in a bird, or new colour spots in a butterfly, make their appearance when a group of determinants has been able to go on varying in the same direction for a long time unimpeded, that is, without being eliminated as injurious by natural selection or obliterated by crossing. This is very likely to happen in the case of an isolated area, and as soon as the conspicuous character thus brought about makes its appearance, sexual selection takes control of it, and ensures that all the individuals, that is, all the germ-plasms which possess it, have the preference in reproduction.
I believe, therefore, that a large number of the endemic species of birds and butterflies in isolated regions result from amixia based upon germinal selection, whose results have been emphasized by sexual selection. Experience corroborates this, as far as I can see, for many of the endemic species of birds in the Galapagos and other islands differ from one another solely or mainly in their colouring, and in many it is especially the males which differ greatly.
As to the humming-birds we may say, without going into details regarding their sexual characters and their distribution, that the many endemic species which inhabit the Alpine regions of isolated South American volcanic mountains differ from one another chiefly in the males and in the secondary sexual characters of these. The family of humming-birds is characteristically Neotropical, that is, it has its centre in the Tropics of the New World, and by far the greater number of humming-bird species—there are about a hundred and fifty—occur there only, while a few occur as migrants north of the Tropical zone, and visit the United States as far north as Washington and New York. We know that many of the most beautiful species have quite a small area of distribution, that many are restricted to a single volcanic mountain, living in the forests which clothe its sides. These species are isolated there, for they do not migrate; apparently they cannot endure the climate of the plains, but remain always in their mountain forests. Without doubt they originated there, chiefly, I am inclined to think, through the variation of the males due to sexual selection. Any one who has seen Gould's magnificent collection of humming-birds in the British Museum in London knows what a surprising diversity of red, green, and blue metallic brilliance these birds display, what contrasts are to be found in the diverse colour-schemes, and what differences they exhibit in the length and form of the feathers of the head, of the neck, of the breast, and especially of the tail. There are wedge-shaped, evenly truncate, and deeply forked tails, some with single long, barbless feathers, and so on. All these characters are confined to the males, and are at most only hinted at in the female; in no species does the female even remotely approach the male in brilliance or decorativeness of plumage.
I do not believe that so many species with very divergent plumage in the males could have developed if they had all lived together on a large connected area. But here, distributed over a large number of isolated mountain forests, the decorative colouring or the distinctive shape which chances to arise through germinal selection on any of these terrestrial islands can go on increasing, undisturbed by crossing with individuals of the ancestral species, and furthered, moreover, by sexual selection.
In this way, if I mistake not, numerous new species have arisen as a result of isolation, and it is quite intelligible that several new species may have arisen from one and the same ancestral species, as we may see from the nearly related yet constantly different species of mocking-bird on the different islands of the Galapagos group.
A number of similar examples might be given from among birds. Thus Dixon calls attention to the species of the thrush genus Catharus, twelve of which live in the mountain forests of Mexico and of South America as far as Bolivia, all differing only slightly from one another and all locally separated. They came from the plains, migrated to the highlands, were isolated there, and then no longer varied together all in the same direction, but each isolated group evolved in a different direction according to the occurrence of chance germinal variations: one developed a chestnut-brown head, another a slate-grey mantle, a third a brown-red mantle, and so on. From what we have already seen in regard to the importance of sexual selection in evolving the plumage of birds, it is probable that this factor has been operative in this case also.
Another example is afforded by the weaver-birds (Ploceus) of South Africa, those ingenious singing-birds resembling blackbirds in size and form, whose pouch-shaped nests, hanging freely from a branch, usually over the water, and with their little openings on the under side, are excellently protected from almost every form of persecution. These birds have in South Africa split up into twenty or more species, but the areas of each are not sharply isolated, and the division into species cannot, therefore, be due to isolation. But it is not difficult to guess upon what it depends, when we know that the males alone are of a beautiful yellow and black colour, while the females are of a greenish protective colouring all over.
Thus, in my opinion, sexual selection plays a part more or less important in the origin of the numerous endemic species of diurnal Lepidoptera which are characteristic especially of the islands of the Malay Archipelago, and which make the Lepidopteran fauna there so rich in individuality. A large number, indeed the majority of the types of Papilionidæ, have a peculiar species, a local form, on most of the larger islands, which is sharply and definitely distinguished from those of the other islands, usually in both sexes, but most markedly in the much more brilliantly coloured males.
Thus each of these types forms a group of species, each of which is restricted to a particular locality, and has usually originated where we now find it, although of course the diffusion of one of these large strong-flying insects from one island to the other is in no way excluded. As an example we may take the Priamus group, the blackish yellow Helena group, the blue Ulyssus group, and the predominantly green Peranthus group.
If we inquire into the causes of this divergence of forms and their condensation into numerous species, we shall find that their roots lie in this case, as in that of all transformations, in germinal selection and the variational tendencies resulting therefrom, but we must regard their fixation us the result of isolation, which prevented the variational tendencies which happened to develop on any one island from being neutralized and swamped by mingling with the variations of other islands. But that sexual selection took control of these striking colour-variations and increased them still further is obvious from the rarely absent dimorphism of the sexes. Even if the females do not consciously select mates from among the males, they will more readily accept as a mate the one among several suitors which excites them most strongly. And that will be the one which exhibits the most brilliant colours or exhales the most agreeable perfume, for we know from their behaviour in regard to flowers how sensitive butterflies are to both these influences.
Although isolation has an important rôle in the formation of all these species, it seems to me an exaggeration to maintain, as many naturalists do, that the splitting up of a species is impossible without isolation. Certainly the splitting up of species is, in numerous cases, facilitated by isolation, and indeed could only have been brought about in its present precision by that means, but it is underestimating the power of natural selection not to credit it with being able to adapt a species on one and the same area to different conditions of life, and we shall return to this point later on in a different connexion. But in the meantime it must suffice to point out that the polymorphism of the social insects affords a proof that a species may break up into several forms in the same area through the operation of natural selection alone.
I am therefore of opinion, with Darwin and Wallace, that adaptation to new conditions of life has, along with isolation, had a material share in the evolution of the large number of endemic species of snail on the oceanic islands. This brings us to the co-operation of natural selection and isolation. If, thousands of years ago, by one of the rarest chances, an Achatina-like snail was carried by birds to the Sandwich Islands, it would spread slowly, at first unvaried, from the spot where it arrived over the whole of the snailless island. But during this process of diffusion it would frequently come in contact with conditions of life which would not prevent it from penetrating further, but to which it was imperfectly adapted, and in such places a process of transformation would begin, which would consist in the fostering of favourably varying individuals, and which would run its course quietly by means of personal selection, based upon the never-ceasing germinal selection, and unhindered by any occasional intrusion of still unvaried members of the species from the original settlement on the island. But these new conditions were not merely different from those of the ancestral country; the island region itself presented very diverse conditions, to which the snail immigrant had to adapt itself in the course of time, as far as its constitution allowed. Terrestrial snails are almost all limited to quite definite localities with quite definite combinations of conditions; none of our indigenous species occurs everywhere, but one species frequents the woods, another the fields; one lives on the mountains, another in the valleys; one on gneiss soil, another on limy soil, a third on rich humus, a fourth on poor river-sand; one in clefts and hollows among damp moss, and another in hot, dry banks of loess, and so on. Although we cannot see in the least from the structure of the animal why this or that spot should be the only suitable one for this or that species, we may say with certainty that each species remains permanently in a particular place because its body is most exactly adapted to the conditions of life there, and therefore it remains victorious in the competition with other species in that particular spot.
In this way the immigrants to the Sandwich Islands must have adapted themselves in the course of time to their increasingly specialized habitats, and in doing so have divided up into increasingly numerous forms, varieties, and species, and indeed into several genera.
But this alone is not sufficient to explain the facts. According to Gulick's valuable researches there live on one little island of the Sandwich group no fewer than 200 species of Achatinellidæ, with 600-700 varieties! This remarkable splitting up of an immigrant species is regarded by him as a result of the isolation of each individual species and variety, and I do not doubt that this is correct as far as a portion of these forms is concerned, and that isolation plays a certain part in regard to them all. Gulick, who lived a long time upon the island, attempts to prove that the habitats of all these nearly related varieties and species are really isolated as far as terrestrial snails are concerned; that intermingling of the snails of one valley with those of a neighbouring one is excluded, and that the varieties of the species diverge more markedly from one another in proportion as their habitats are distant. On the other hand, species of different genera of Achatinellidæ often live together on the same area; but they do not intermingle.
Although Gulick's statements are worthy of all confidence, and though his conclusions have great value as contributions to the theory of evolution, I do not think that he has exhausted the problem of the causes of this remarkable wealth of forms among the terrestrial snails of oceanic islands. It is not that I doubt the relative and temporary isolation of the snail-colonies at numerous localities in the island of Oahu. But why have we not the same phenomenon in Germany, in England or Ireland? Gulick anticipates this objection by pointing out the peculiar habits of the Oahu snails. Many of the species there are purely arboreal animals, living upon trees and never leaving them, even during the breeding season, or in order to deposit eggs, for they bring forth their young alive. Active migration from forest to forest seems excluded by the fact that on the crests of the mountains there is a less dense forest of different kinds of trees, and dry sunny air, which could not be endured by the species of Achatinella and Bulimella, which love the moist shades of the tropical forests. Active migration over the open grass-land at the mouths of the valleys is also excluded.
It must be admitted that the isolation of these forest snails in their valleys is for the time being very complete, and that intermingling of two colonies which live in neighbouring valleys does not occur by active migration, within the span of one or several human generations. It will also be admitted that our terrestrial snails in Central Europe are much less isolated in their different areas, that, for instance, they could get from one side of a mountain to the other by active migration; but we must nevertheless repeat the question: how does it happen that in Oahu every forest, every mountain-crest, and so on, has its own variety or species, while our snails are distributed over wide stretches of country, frequently without even developing sharply defined local varieties? The large vineyard or edible snail (Helix pomatia) occurs from England to Turkey, that is, over a distance of about 3,000 kilometres, and within this region it is found in many places which might quite as well be considered isolated as adjacent forest valleys in Oahu. It occurs also on the islands of the Channel and of the Irish Sea, and lives there without intermingling with the members of the species on the mainland. But even on the Continent itself it would be possible to name hundreds of places in which they are just as well protected from intermingling with those of other areas as they are in Oahu. There too the snails must somehow have reached their present habitat some time or other, perhaps rather in an indirect way, by means of other animals; but this is true also of the snails of a continent, as we shall show more precisely later on. In the meantime let us assume that this is so, and that the vineyard snail (Helix pomatia), or some other widely distributed snail, is relatively isolated. Why then have not hundreds of well-marked varieties evolved—a special one for each of the isolated areas?
Obviously there must have been something in operation in the Sandwich Islands which is absent from the continental habitats of Helix pomatia, for this species shows fluctuations only in size, but is otherwise the same everywhere, and the few local varieties of it which occur are unimportant. I am inclined to believe that this 'something' depends on two factors, and especially on the fact that the immigrant snail enters upon a period of variability. This will be brought about in the first place by the fact that the climate and other changes in the conditions of life will call forth a gradually cumulative disturbance in the equilibrium of the determinant system, and thus a variability in various directions and in various combinations of characters. To this must be added the operation of natural selection, which attempts to adapt the immigrant to many new spheres of life, and thus increases in diverse ways the variational tendencies afforded by germinal selection. These two co-operating factors bring the species into a state of flux or lability, just as a species becomes more variable under domestication, likewise as a direct effect of change of food and other conditions, such as the consciously or unconsciously exercised processes of selection. It follows from this that, in the gradual diffusion of snails all over the island, similar localities would almost never be colonized by exactly similar immigrants, but by individuals containing a different combination of the existing variations, so that in the course of time different constant forms would be evolved through amixia in relatively isolated localities.
But everything would be different in the diffusion of a new species of snail in a region which was already fully or at least abundantly occupied by snail-species. Let us leave out of account altogether the first factor in variation, the changed climate, and we see that a species in such circumstances would have no cause for variation, because it would find no area unoccupied outside of the sphere to which it was best adapted; it would therefore not be impelled to adapt itself to any other, and in most cases could not do so, because in each it would have to compete with another species superior to it because already adapted.
The case would be the same if an island were suddenly peopled with the whole snail-fauna of a neighbouring continent, with which a land connexion had arisen. If the island had previously been free from snails, all the species of the mainland would be able to exist there in so far as they were able to find suitable conditions of life, but each species would speedily take complete possession of the area peculiarly suited to it, so that none of their fellow migrants would be impelled, or would even find it possible, to adapt themselves to new conditions and thus to become variable and split up into varieties. If Ireland were at present free from snails, and if a land connexion between it and England came about, then the snail-fauna of England would probably migrate quite unvaried to Ireland, and in point of fact the snail-fauna of the two islands, which were formerly connected, is almost the same. For the same reason the fauna of England, as far as terrestrial snails are concerned, is almost the same as that of Germany.
On the other hand, it may be almost regarded as a law that an individual migrant to virgin territory must become variable. This could not be better illustrated than by the geographical distribution of terrestrial snails, which emphasizes the fact that a striking wealth of endemic species is to be found on all oceanic islands. Moreover, the fact that the number of these endemic species is greater in proportion to the distance of the island from the continent, indicates that the variability sets in more intensively and lasts longer in proportion to the small number of species which become immigrants in the island, and in proportion to the number of unoccupied areas which are open to the descendants of the immigrant species. This is undoubtedly the reason why the Sandwich Islands do not possess a single species which occurs elsewhere, and the segregation of the unknown ancestral form into many species and several (four) sub-genera is also to be interpreted in the same way. There was probably in this case only one immigrant species, which found a free field, and adapted itself in its descendants to all the conditions of snail-life which obtained there, and in doing so split up into numerous and somewhat markedly divergent forms. But the number of different forms is much greater than the number of distinctive habitats, as Gulick indicates and substantiates in detail, for similar areas, if they are relatively isolated from one another, are inhabited not by the same forms, but by different though nearly related varieties, and this depends on the fact that from the species which was in process of varying a different combination of variations would be sent out at different periods, and the temporary isolation would result in the evolution of special local varieties.
But I do not believe that this would continue for all time. I rather think that these—let us say—representative varieties would diminish in numbers in the course of a long period. For the isolation of single valley-slopes or of particular woods is not permanent, individuals are liable to be carried from one to another in the course of centuries as they were at the beginning of the colonization of the isolated woods; forests are cleared or displaced by geological changes, connexions are formed between places, which were formerly separated, and in the course of another geological period the number of representative varieties, and probably even of species, will have diminished considerably,—the former will have been fused together, the latter in part eliminated. Even now Gulick speaks regretfully of the decimation of rare local forms by their chief enemies, the mice.
But even if the number of endemic forms in insular regions diminishes from the time when they were first fully taken possession of, it nevertheless remains a very high one, for even now Madeira possesses 104 endemic terrestrial snails, the Philippines have more than the whole of India, and the Antilles as many as the whole American continent.
Many naturalists believe that each isolated variety must diverge further and further from its nearest relatives as time goes on. Although I entirely admit that this is possible, for I have endeavoured to show that variational tendencies which have once arisen in the germ-plasm go on in the same direction until they are brought to a full stop in some way or other, yet I cannot admit that this must always be so. The species which has been carried to a strange area need not always contain particular variational tendencies in its germ-plasm, and need not in every case be impelled to such variations by the influence of new conditions. We know species which have made their way into new regions, and, without varying at all, have held their own with, or even proved superior to, the species which were already settled there. Many cases of this kind are known, both among plants and animals; these have been brought by man, intentionally or by chance, from one continent to another, and have established themselves and spread over the new area. I need only recall the evening primrose (Œnothera biennis[28]), whose fatherland is Virginia, but whose beautiful big yellow blossoms now display themselves beside nearly every river in Germany, having migrated stream-upwards along the gravelly soil; or the troublesome weed (Erigeron canadense), which is now scarcely less common in our gardens than in those of Canada; or the sparrow (Passer domesticus), which was introduced into the United States to destroy the caterpillars, but which preferred instead to plunder the rich stores of corn, and in consequence of these favourable conditions increased to such an extent that it has now become a veritable pest, all imaginable means for its extirpation having been tried—as yet, however, with no great results.
[28] This was written before the appearance of the researches which De Vries has made on the variations of Œnothera in Europe. Thus the illustration may not be quite apposite, for it seems to remain undetermined whether the 'mutations' which occur in Holland do not also occasionally appear in America. See end of lecture xxxiii.
In all these cases the migration is certainly of recent date, and it is quite possible that, when a longer time has elapsed, some variations will take place in the new home, but in any case these instances prove that an immigrant species can spread over its new area without immediately varying.
Similarly, it must be admitted that species which have belonged to two continents ever since Tertiary times need not have diverged since that time, and we know, for instance, thirty-two species of nocturnal Lepidoptera which are common to North America and to Europe and yet exhibit no differences, while twenty-seven other nocturnal Lepidoptera are, according to Grote, represented in America by 'vicarious' species, that is, by species which have varied slightly in one or other of the two areas, perhaps in both.
To sum up: we must undoubtedly admit that isolation has a considerable influence in the evolution of species, though only in association with selection in its various grades and modes, especially germinal selection, natural selection, and sexual selection. We can say generally that each grade and mode of selection will more readily lead to the transformation if it be combined with isolation. Thus germinal selection may call forth slight divergences in colour and marking, which will be permanent if the individuals concerned are in an isolated region. In isolation these variations will increase undisturbed, and in some circumstances will be intensified by sexual selection, so that the male sex will vary alone in the first place, though the female may follow, so that ultimately the whole species will be transformed. Finally, the most marked effect of isolation is seen when individual members of a species are transferred to virgin territory which offers unoccupied areas, suitable not to one particular species alone, but to many nearly related species, so that the immigrant colony can adapt itself to all the different possibilities of life, and develop into a whole circle of species. But we saw that such an aftergrowth of new forms, whether varieties, species, or even genera, may far exceed the number of different kinds of localities, if there be relative isolation between the different groups of immigrants within the insular region, as happens in the case of slow-moving animals like the terrestrial snails, or of small singing-birds, to which each island of a little archipelago is a relatively isolated region (Galapagos).
We may thus fully recognize the importance of local isolation without regarding the absence of crossing with the members of the species in the original habitat as the sole cause of species-formation, without setting 'isolation' in the place of the processes of selection. These last, taken in the wide sense, always remain the indispensable basis of all transformations, but they certainly do not operate only in the form of personal selection, but, wherever indifferent characters are concerned, in that of germinal selection. Here, too, we see the possibility of reconciliation with those naturalists who regard transformations as primarily dependent upon internal forces of development. The fact is that all variations depend upon internal causes, and their course must be guided by forces which work in an orderly way. But the actual co-operation of all these forces and variations is not predetermined, but depends to a certain extent upon chance, for of the possible modes of evolution the one which gains the upper hand in the play of forces at the moment is alone followed, the better are everywhere preferred, from the most minute vital units of the germ-plasm, up to the struggle between individuals and between species.