It was the observance of this fact which led Gulick to insist with such emphasis on the importance of geographical isolation as a factor in evolution. He discovered that the land mollusca of the Sandwich Islands fall into a great number of varieties. 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.