With the exception of Mr. Gulick, I cannot find that any other writer has hitherto stated this supremely important distinction between isolation as discriminate and indiscriminate. But he has fully as well as independently stated it, and shown in a masterly way its far-reaching consequences. Indiscriminate isolation he calls Separate Breeding, while discriminate isolation he calls Segregate Breeding. For the sake, however, of securing more descriptive terms, I will coin the words Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate isolation, or separate breeding. Homogamy, on the other hand, answers to discriminate isolation, or segregate breeding: only individuals belonging to the same variety or kind are allowed to propagate. Isolation, then, is a genus, of which Apogamy and Homogamy are species[4]. [4] I may here most conveniently define the senses in which all the following terms will be used throughout the present discussion:--_Species_ of isolation are, as above stated, homogamy and apogamy, or isolation as discriminate and indiscriminate. _Forms_ of isolation are modes of isolation, such as the geographical, the sexual, the instinctive, or any other of the numerous means whereby isolation of either species may be secured. _Cases_ of isolation are the instances in which any of the forms of isolation may be at work: thus, if a group of _n_ intergenerants be segregated into five groups, _a_, _b_, _c_, _d_, _e_, then, before the segregation there would have been one case of isolation, but after the segregation there would be five such cases. Now, in order to appreciate the unsurpassed importance of isolation as one of the three basal principles of organic evolution, let us begin by considering the discriminate species of it, or Homogamy. To state the case in the most general terms, we may say that if the other two basal principles are given in heredity and variability, the whole theory of organic evolution becomes neither more nor less than a theory of homogamy--that is, a theory of the causes which lead to discriminate isolation, or the breeding of like with like to the exclusion of unlike. For the more we believe in heredity and variability as basal principles of organic evolution, the stronger must become our persuasion that discriminate breeding leads to divergence of type, while indiscriminate breeding leads to uniformity. This, in fact, is securely based on what we know from the experience supplied by artificial selection, which consists in the intentional mating of like with like to the exclusion of unlike.