Chapter XIV: Part 14

But it would lead me too far if I were to enter on a detailed discussion of these cases, which have not yet been fully worked up; only one thing more need be mentioned, that is, that the aberrations induced by cold are to a certain extent transmissible. Standfuss first succeeded in making some aberrant specimens of _Vanessa urticæ_ reproduce, and from their eggs he procured butterflies which showed a much slighter deviation from the normal, which however was still so decided that it could not be regarded as due to chance. I myself succeeded in doing the same, but the deviation in this case was much slighter. But that these observed cases are rightly referred to the cold to which their parents had been subjected is proved by other observations recently published by E. Fischer. These refer to one of the Bombycidæ (_Arctia caja_), which flies by day, and accordingly has a gay and very definite marking and coloration. A large number of pupæ were exposed to cold at 8° C., and some of these resulted in striking and very dark aberrant forms (Fig. 129, _A_). A pair of these yielded fertilized eggs; in the progeny, which were reared at a normal temperature, there were among the much more numerous normal forms a few (17) which exhibited the aberration of the parents, though to a considerably less degree (Fig. 129, _B_).

This shows that the cold had affected not only the wing-primordia of the parental pupæ, but the germ-plasm as well, and at the same time that this latter variation was less marked than that of the determinants of the wing-rudiments. This gives rise _to an appearance_ of the transmission of acquired characters.

In the case of many of these cold-aberrations in Lepidoptera the cold gives rise to variations, but does so not by creating anything new, but by giving the predominance to primary constituents which have long been present, but are usually suppressed, and so it is also among the plants. I have in mind, for instance, the interesting experiments of Vöchting on the influence of light in the production of flowers in phanerogams. These showed that the common balsam (_Impatiens noli me tangere_) produces its familiar open flowers in a strong light, but in weak light only bears small, closed, so-called 'cleistogamous' flowers. But it would be utterly erroneous to suppose that the strong or weak light is the real cause, the _causa materialis_, of these two forms of flowers: the degree of illumination is merely the stimulus which provokes one or other of the primary constituents to development, both kinds being present in the constitution of the plant. As has long been known, the balsam normally possesses two kinds of flowers, and the slumbering primary constituents of these are so arranged that the open flowers develop where there is a prospect of insect visits and cross-fertilization, that is, in sunny weather or in a strong light, while closed and inconspicuous flowers adapted for self-fertilization develop in weak light, that is, in shady places and in concealed parts of the plant, where insect visits are not to be expected.

Among plants we find thousands of instances of such reactions of the organism to external stimulus--reactions which are not of a primary nature, that is, are not the inevitable consequences of the plant's constitution, but which depend upon adaptations of the special constitution of a species or group of species to the specific conditions of its life. To this category belong all the phenomena of heliotropism, geotropism, and chemotropism, which have been discovered by the numerous and excellent observations of the plant physiologists. That all these are adaptations and secondary reactions to stimuli is proved by the fact that the same stimuli affect the homologous parts of different species in very different, and often in opposite ways. For instance, while the green shoots of most plants turn towards the light, being positively heliotropic, the climbing shoots of the ivy and the gourd are negatively heliotropic, which is an adaptation to climbing. In this case the reason of the difference in the mode of reaction must lie in the difference of constitution of the cellular substance of the shoot, and since this may differentiate so very diversely in its relation to light, the power of reaction which plant substance in general has to light must not be regarded as a primary character, like the specific gravity of a metal or the chemical affinities of oxygen and hydrogen, but as adaptations of the living and varying substance to the special conditions of life. And the origin of these adaptations must depend upon processes of selection, and on these alone. This is just the difference between living and non-living matter,--that the former is variable to a high degree, the latter is not; it is the fundamental difference upon which the whole possibility of the origin of an animate world depends.

Among animals also we must distinguish between the direct effects of external influence to which the organism is not already adapted, and those reactions which imply a previously established adjustment to the stimulus. That is, we must distinguish between primary and secondary reactions.

For instance, Herbst made artificial sea-water in which the sodium was partially replaced by lithium, and the eggs of sea-urchins developed in this artificial sea-water into very divergent larvæ of peculiar structure. We have here a primary reaction of the organism to changed conditions of life--not an adaptation, not a prepared reaction. Accordingly these 'lithium larvæ' eventually perished.

The increasing blackness of _Polyommatus phlæas_, which we have already discussed, must also be regarded as a primary reaction, but not so the variations--often misinterpreted--of those species of _Artemia_ which live in the brine-pools of the Crimea, in regard to which Schmankewitsch showed that, when the amount of salt in the water is diminished, they undergo certain changes which bring them nearer to the fresh-water form _Branchipus_, while when the salt is increased in amount they vary in the contrary direction. Probably these are adaptations to the periodically changing salinity of their habitat.

There can be no doubt of this in the case of the caterpillars of different families, in regard to which Poulton showed that in their early youth they possess the power of adapting themselves exactly to the colour of their chance surroundings. It is obvious that the protection which the caterpillar would gain from being coloured _approximately_ like its surroundings would be insufficient, for instance because the surroundings may be very diverse, since the species lives upon different, variously coloured plants and plant-parts. Thus a facultative adaptation arose. Selection gave rise to an extraordinarily specialized susceptibility on the part of the different cell elements of the skin to differences of light, and the result of this is that the skin of the caterpillar invariably takes on the colouring which is reflected upon it in the first few days of its life from the plants and plant-parts by which it is surrounded. Thus the caterpillars of one of the Geometridæ, _Amphidasis betularia_, take on the colours of the twig between and upon which they sit, and they can be made black, brown, white, or light green quite independently of their food, according to the colour of the twigs (or paper) among which they are reared.

Colour-change in fishes, Amphibians, Reptiles, and Cephalopods, depends upon much more complex adaptations. In their case a reflex-mechanism is present which conducts the light-stimulus affecting the eye to the brain, and there excites certain nerves of the skin; these in their turn cause the movable cells of the skin which condition the colouring to change and rearrange themselves in the manner necessary to bring about the harmonization of colour. On this depends the colour-change of the famous chamæleon, and also the scarcely less striking case of the tree-frog, which is light green when it sits on trees, but dark brown when it is kept in the dark. All these are secondary reactions of the organism in which the external stimulus is, so to speak, made use of to liberate adaptive variations, either permanently or transitorily. In the caterpillars colour-changes are permanent, that is, it is only the young caterpillar which takes on the colour of its surroundings; later it does not change, even when it is exposed to different light, or intentionally placed upon a food-plant of a different colour. In fishes, frogs, and cuttlefishes, on the contrary, the reaction of the colour-cells to light only lasts a little longer than the light-stimulus, and it changes with it. The purposiveness of this difference of reaction is obvious.

We cannot say to what degree the direct influence of external conditions is effectively operative on the germ-plasm, or how far, by persistently repeated slight changes, the determinants and the parts of the body determined by them may be made to vary in the course of generations; that is to say, how large a part this direct influence of climate and food may play in the transmutation of species. We can give no answer from experience, because there is an entire lack of perfectly satisfactory and clear experiments; we only know in a few cases how great the variations are which can be brought about in the body during the individual life by means of any of these factors. In most cases it is uncertain whether actually hereditary effects play any part, that is, whether the germ-plasm itself is affected. But if we wish to be theoretically clear as to how far direct climatic effects may go, we may say this, that they may operate as long as they cause no disturbance in the life of the species concerned, for at the moment that such a direct effect begins to be prejudicial to the species personal selection will step in, and, by preferring the individuals which react least strongly to the climatic stimulus, will inhibit the variation. If in any case this should be physically impossible, the species would die out in the climate in question. That a species of plant or animal has climatic limits indicates that individuals which go beyond these are exposed to influences which make life impossible and which natural selection is unable to neutralize. We are here brought face to face with one of the limits to the scope of natural selection. There is no doubt that the influences of the environment must always have a powerful effect upon the soma of the individual, but we have seen, in the case of Alpine plants and of galls, how very far this effect may go without leaving any trace in the germ-plasm.

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.