SELF-CUMULATIVE ENDOWMENTS. 97 
external conditions tends to obscure, though not to deny, the fact 
that the breeding together of the better adapted which causes the 
Increase of adaptation is due to the different degrees of endowment 
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 both belong in different degrees 
to members of the same intergenerant that are equally fitted. The 
cumulation 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. The increase of segregative 
endowments and of fertility when codperating is due to principles 
quite different from this, and differing from each other. The segre- 
galive endowments augment through the inherent tendency of those 
more highly endowed in this respect to breed exclusively with those 
of the same form, and, therefore, in the long run to segregate from 
others; while the fertility of the more fertile neither prevents the 
individual success of 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 
the breeding together of those possessing higher degrees of the special 
endowment, and so increase the average endowment, either of the 
whole number of the offspring or of the segregated portion. Vigor 
increases through the breeding together of the more vigorous, result- 
ing from their overcoming and crowding out the less vigorous without 
allowing them full opportunity to propagate, though they are adapted 
to conditions lying in the environment. Adaptation increases 
through the breeding together of the better adapted, resulting from 
the failure of the less adapted individuals to live and thrive. Segre- 
gative endowments increase through the breeding together of the more 
highly endowed, resulting from the fact that as long as segregation is 
incomplete more than half of each generation of pure descent are 
necessarily the offspring of parents whose segregative endowments 
were above the average. Fertility increases through the breeding 
together of the more fertile, resulting from the fact that more than 
half of each generation are the offspring of parents of more than 
average fertility. Among those that are equally adapted to the envi- 
ronment the ratio of propagation varies directly as the ratio of fer- 
tility. This propagation according to degrees of fertility 1s what I call 
the law of cumulative fertility, through fecundal selection. It is not 
due to different degrees of success, or to any advantage which the 
