represent two species, and the line between represents their specific differences; _r r′_ represent different races or permanent varieties; _v v′_ two strong varieties; _d d′_ ordinary individual differences; _c c′_ close resembling or consanguineous individuals. The undulating line represents the effect of crossing these various kinds. It is seen that “in-and-in breeding,” _c c′_, produces bad effect (negative ordinates); breeding of ordinary individual differences, _d d′_, keeps the stock at the ordinary level--in its typical form; crossing two strong varieties, _v v′_, produces maximum good effect (positive ordinates); crossing decided races produces again bad effects, which become infinitely bad as we approach species, S S′.[31] [Illustration: FIG. 69.] It is generally admitted that long-continued very close breeding has a bad effect. Even in plants, Darwin has shown that cross-fertilization has better effect than self-fertilization, this last being of course the closest possible breeding. But it is probable that the principal bad effect is not on the stock but _on the process of evolution_. Very close breeding weakens the stock, ordinary breeding of individual differences maintains the stock at the ordinary level and fixes it. Cross-breeding of varieties strengthens the stock, and also (and this is its main advantage) produces plasticity in the stock, gives rise to strong divergent variations, or even sports, and thus becomes a main agent in evolution. It is probable, moreover, that the higher the function the more sensitive is it to these effects of breeding. Therefore, the effect is greater in man than in any other animal. It is true that many have doubted the bad effect of close breeding in man, and have brought forward formidable statistics to substantiate their position; but these doubtless take no account of the most important function, the psychic, and especially the most important element in every function, so far as evolution or progress is concerned, viz., _plasticity_ or capability of progressive improvement. The tendency of consanguineous breeding, or even the breeding of persons of like character and experiences, as in an isolated community, is, if not to deteriorate the physique, at least to fix, stereotype the character, and thus to check social progress. Contrarily, the crossing of varieties of the same race seems not only to strengthen but, by the diverse inheritance, to produce plasticity of character and capacity for progress. But the difference between the primary races seems too great for crossing with advantage. Some degree of sexual repugnance which undoubtedly exists between the primary races is the psychical sign of this fact.[32]