The Evolution Theory, Vol. 2 of 2

by August Weismann

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.