through: cumulative segregation. 
247 
they belong in different degrees to members of the same Inter- 
generant that are equally fitted. The cumulation of vigour, as 
that of adaptation, is, I think, rightly classed as a form of Selec- 
tion ; 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 segre- 
gative endowments and of fertility is due to principles quite 
different from this, and differing from each other. The segre- 
gative endowments augment 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 fertility of the more fer- 
tile 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 the breeding together of those possessing higher degrees 
of the special endowment, and so increase the average endow- 
ment, either of the whole number of the offspring, or of the 
segregated portion. Vigour increases through the breeding to- 
gether of the more vigorous, resulting from their overcoming and 
crowding out the less vigorous without allowing them full oppor- 
tunity to propagate. Adaptation increases through the breeding 
together of the better adapted, resulting from their supplanting 
their rivals without allowing them full opportunity to propagate. 
Segregative 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 segre- 
gative 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. As the breeding to- 
gether of the more vigorous and the better adapted, caused by their 
superior success, tends to increase and intensify the vigour and 
adaptation of successive generations, so the breeding together of 
those more highly endowed with Segregative powers, caused by 
the Segregation, tends to strengthen and intensify the Segregative 
powers in successive generations ; and so the breeding together of 
the more fertile, caused by the larger proportion of offspring 
produced by the more fertile, tends to increase the fertility of 
