interesting and very important kind of variation is known to occur in some individuals of the human species. It is well known that a man and woman have been cross-sterile with one another, being unable to have children; yet when separating and mating with others they have both been cross-fertile, families being reared by both of them. Variations are commencing species; isolated variations diverge more and more into distinct species. This fact, then, of the occurrence in nature of variations in some of the individuals of a species by which they are cross-fertile with one another, but cross-sterile with other varieties and with the parental stock, shows that Nature has a most effectual means by which varieties may be isolated from one another,—just as effective means as man, the breeder of varieties of domestic animals, possesses in isolating these domestic varieties by physical barriers, as fences, etc. Cross-sterility, therefore, in nature, is a most effective sexual barrier. The special form of it under consideration is what Romanes has called Physiological Selection. The varieties that are isolated by this sexual barrier have got to run the risks of in and in breeding, which Darwin has shown occur in domesticated animals, but which Wallace thinks are much less in a state of nature. Another very important mode of Nature for isolating varieties is that which arises out of the instinctive preferences of animals. There is a tendency for like to breed with like where varieties are formed. The pale and dark colored herds of fallow deer in the Forest of Dean have never been known to interbreed. In the Falkland Islands all the cattle are known to have descended from the same stock. Here there are differently colored herds of cattle, and those cattle of the same color will interbreed with each other, but not with individuals outside their own color-caste, as Morgan expresses it. When two flocks of heath sheep and merino sheep are mixed together, they do not interbreed. This isolation of varieties by instinctive preferences for those individuals with like color patterns may be spoken of as _preferential mating_ through _recognition marks_. A very obvious mode of isolation in nature is by geographical barriers (including migration). In treating of environment we learned that during the geologic ages of the globe, the physical geography and climate have repeatedly changed. A very cold temperature, a mountain chain, a body of water, a stretch of desert land, may completely prevent interbreeding, on either side of the barrier, between the individuals of a species.