The only other directive evolution-factor that biologists are at all agreed about, besides selection, is isolation--a general term for all the varied ways in which the radius of possible intercrossing is narrowed. As expounded by Wagner, Weismann, Romanes, Gulick, and others, isolation takes many forms--spatial, structural, habitudinal, and psychical--and it has various results.
It tends to the segregation of species into subspecies, it makes it easier for new variations to establish themselves, it promotes prepotency, or what the breeders call "transmitting power," it fixes characters. One of the most successful breeds of cattle (Polled Angus) seems to have had its source in one farmsteading; its early history is one of close inbreeding, its prepotency is remarkable, its success from our point of view has been great. It is difficult to get secure data as to the results of isolation in nature, but Gulick's recent volume on the subject abounds in concrete illustrations, and we seem warranted in believing that conditions of isolation have been and are of frequent occurrence.
Reibmayr has collected from human history a wealth of illustrations of various forms of isolation, and there seems much to be said for his thesis that the establishment of a successful race or stock requires the alternation of periods of inbreeding (endogamy) in which characters are fixed, and periods of outbreeding (exogamy) in which, by the introduction of fresh blood, new variations are promoted. Perhaps the Jews may serve to illustrate the influence of isolation in promoting stability of type and prepotency; perhaps the Americans may serve to illustrate the variability which a mixture of different stocks tends to bring about. In historical inquiry into the difficult problem of the origin of distinct races, it seems legitimate to think of periods of "mutation"--of discontinuous sporting--which led to numerous offshoots from the main stock, of the migration of these variants into new environments where in relative isolation they became prepotent and stable.[94]
The biological use of the term "isolation" introduces a new emphasis. Separation may be spatial, but its effects are increasingly structural and functional. Indeed, spatial isolation was a factor in the origin of species because of specialized organic adaptation to varied geographic conditions. In other words, the structure of the species, its habits of life, and its original and acquired responses, tend to isolate it from other species.