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DIVERGENT EVOLUTION THROUGH SEGREGATION. 315 
same species or variety results in higher fertility than the crossing of 
different species or varieties. In like manner segregate vigor is the 
relation in which species or varieties stand to each other when the in- 
ter-generation of members of the same species or variety produces off- 
spring more vigorous than those produced by crossing with other species 
or varieties. Integrate fecundity and integrate vigor are the terms 
by which I indicate the relation to each other of forms in which the 
highest fertility and vigor are produced by crossing, and not by inde- 
pendent generation. 
Before discussing these principles through which the influence of 
segregation is greatly increased, it will be an advantage if we can gain 
some idea of the nature of cumulative fertility in its relations to a law 
of still wider import. i refer to the fourfold law of antagonistic in- 
crease and mutual limitation between (1) integration, (2) segregation, 
(3) adaptation, (4) multiplication—in other words, between (1) general 
invigoration and power of variation through crossing, (2) the opening 
of new opportunities and independent possibilities, (3) special adapta- 
tion to present circumstances, (4) powers of multiplied individualiza- 
tion. Darwin has considered at length the first and third, though I do 
not remember that he has anywhere pointed out that their development 
is due to a kind of self-augmentation. I believe this is so emphatically 
the case that the former might well be called the law of self-cumulative 
vigor, and the latter the law of self-cumulative adaptation. Corre- 
sponding to these two laws, I find the additional laws of self-cumulative 
segregation and self-cumulative fertility. Darwin’s theory, that diver- 
sity of natural selection is directly and necessarily dependent on ex- 
posure to different external conditions tends to obscure, though not to 
deny, the fact that the breeding together of the better adapted, which 
‘auses the increase of adaptation, is due to the different degrees of en- 
dowment in the organism, rather than to diversity in the environment. 
It is also true of segregative endowment and of fertility that they are 
necessarily cumulative whenever they belong in different degrees to 
members of the same intergenerant that are equally fitted. The cum- 
ulation of vigor, as that of adaptation, is I think rightly classed as a 
form of selection; for in both cases it depends on the power of the 
more highly endowed to supplant the less endowed without allowing 
them full opportunity to propagate; but the increase of segregative 
endowments and of fertility is due to principles quite different from 
this, and differing from each other. The segregative endowments aug- 
ment through the inherent tendency of the more highly endowed to 
breed more exclusively with those of the same form, and therefore in 
the long run to breed more exclusively with each other; while the ter- 
tility of the more fertile neither drives out the less fertile nor holds 
the two classes apart, but simply multiplies the offspring of the more 
fertile, making it sure that in each generation they will predominate. 
But all these forms of augmentation correspond, in that they secure 
