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 cooperating is due to principles 

 quite different from this, and differing from each other. The segre- 

 gative 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 is 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 



