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.