These islands are very hilly, and Gulick found that each of the varieties is confined not merely to one island, but to one valley. “Moreover,” writes Romanes, on p. 16 of Darwin and after Darwin, “on tracing this fauna from valley to valley, it is apparent that a slight variation in the occupants of valley 2, as compared with those of the adjacent valley 1, becomes more pronounced in the next, valley 3, still more so in 4, etc., etc. Thus it was possible, as Mr Gulick says, roughly to estimate the amount of divergence between the occupants of any two given valleys by measuring the number of miles between them. . . . The variations which affect scores of species, and themselves eventually run into fully specific distinctions, are all more or less finely graduated as they pass from one isolated region to the next; and they have reference to changes of form or colour, which in no one case presents any appearance of utility.”
Hitherto three different attempts have been made to explain this and allied phenomena:—
1. That it is the result of isolation.
2. That it is the result of natural selection.
3. That it is the result of the action of the environment on the organism.
Let us consider these in inverse order.
Local Species
In the case of some organisms, more especially plants, invertebrates, and fish, the environment does exert a direct influence on their colouration. But, as we have seen, the changes in colour, etc., thus induced appear never to be transmitted to the offspring of the organisms so affected. They disappear when the offspring are removed to other surroundings.
On the other hand, local races or species—as, for example, the white-cheeked variety of sparrow found in India—usually retain their external appearance when the environment is changed. In the one case the peculiarity is not inherited; in the other it is inherited.
The Wallaceian explanation is, of course, that the phenomenon is the result of natural selection. There must, say Wallace and his followers, be some differences in the environment, differences which we poor human beings cannot perceive, that have caused the divergence between the various isolated sections of the species. In the case of some local species this explanation is probably the correct one, but we have no hesitation in saying that natural selection is unable to offer a satisfactory explanation in a considerable number of instances. Take, for example, the case of the land mollusca of the Sandwich Islands. Mr Gulick worked for fifteen years at them, and states that so far as he is able to ascertain the environment in the fifteen valleys is essentially the same. “To argue,” writes Romanes, on p. 17 of vol. iii. of Darwin and after Darwin, “that every one of some twenty contiguous valleys in the area of the same small island must necessarily present such differences of environment that all the shells in each are differently modified thereby, while in no one out of the hundreds of cases of modification in minute respects of form and colour can any human being suggest an adaptive reason therefore—to argue thus is merely to affirm an intrinsically improbable dogma in the presence of a great and consistent array of opposing facts.”