There are several other forms of discriminate isolation. Sexual selection would be one of these. Suppose, for example, that in any species there are large and small varieties formed, and like tends to breed with like, then the small individuals will breed with other small individuals, while large ones will mate with large ones; thus two races—a large one and a small one—will be evolved side by side, provided, of course, natural selection does not step in and destroy one of them. Another kind of discriminate isolation may be due to the fact that one variety is ready to pair before the other; thus two races are likely to arise which breed at different seasons. It is unnecessary for us to discourse further on the subject of discriminate isolation; those interested in the subject should read vol. iii. of _Darwin and after Darwin_, by Romanes. Indiscriminate Isolation It is impossible to deny the importance of discriminate isolation as a factor in evolution. On this there can be no room for disagreement among biologists. It is when we come to the subject of indiscriminate isolation that we enter a region of zoological strife. Is indiscriminate isolation _per se_ a factor of evolution? Romanes, Gulick, and Wagner assert that it is, Wallace and his adherents assert that it is not. As the burden of proof is on the former, they are entitled to the first hearing. “We may well be disposed, at first sight,” writes Romanes (_Darwin and after Darwin_, p. 10), “to conclude that this kind of isolation can count for nothing in the process of evolution. For if the fundamental importance of isolation in the production of organic forms be due to its segregation of like with like, does it not follow that any form of isolation which is indiscriminate must fail to supply the very condition on which all the forms of discriminate isolation depend for their efficacy in the causing of organic evolution? Or, to return to one’s concrete example, is it not self-evident that the farmer who separated his flock into two or more parts indiscriminately, would not effect any more change in his stock than if he had left them all to breed together? Well, although at first sight this seems self-evident, it is, in fact, untrue. For, unless the individuals which are indiscriminately isolated happen to be a very large number, sooner or later their progeny will come to differ from that of the parent type, or unisolated portion of the parent stock. And, of course, as soon as this change of type begins, the isolation ceases to be indiscriminate; the previous apogamy has been converted into homogamy, with the usual result of causing a divergence of type. The reason why progeny of an indiscriminately isolated section of an originally uniform stock—_e.g._ of a species—will eventually deviate from the original type is, to quote Mr Gulick, as follows:—‘No two portions of a species possess exactly the same average character, and the