[page] 406 [Gulick, Diversity of Evolution]

[page] 407

Mr. Wallace agreed with the Rev. J. T. Gulick in his interpretation of facts which appeared to be exceedingly remarkable. He had had the opportunity of working at a limited group of organisms in a small part of the world. The results he had described were a type of what took place over whole continents, and exhibit an example of variation and geographical distributions, perhaps the most remarkable that occurs on the surface of the earth. With the general principle that variation does not depend on difference in external condition, he altogether agreed. He thought in this matter that there was a confusion of two distinct things, even in some cases by Mr. Darwin himself. Variation was confounded with the formation of varieties. That it was not dependent on the change of conditions was evidenced by the fact that the varieties of domestic animals and plants were not due to this cause, but only to advantage being taken of spontaneous variation and identical conditions. Horticulturists obtained new varieties of any plant that was introduced into cultivation by growing it upon a very large scale, and selecting the sports which were sure to occur. In this case variation was accumulated by artificial selection, just as it is accumulated in nature by natural selection. This requires, as a condition of its action, a change of external conditions. We all know that closely allied, though distinct species, were found inhabiting distinct areas—for example, islands; and with large continental areas it was the same. This had led to the very general idea that it was variation of condition over those areas which had produced the varieties, whereas it had merely selected them. In the Sandwich Islands there was no difference of physical conditions adequate to produce this result. This was seen in the number of intervening forms which existed. It seemed due to the absence of any weeding-out effect. The land molluscs had hardly any competitors to struggle with, and no enemies, quadrupeds and reptiles being absent, and birds few. The rivers were small and would only distribute any form through the same valley. All these conditions favoured this remarkable persistence of closely linked forms.