316 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
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, resulting 
from their overcoming and crowding out the less vigorous without 
allowing them full opportunity to propagate. Adaptation inereases 
through the breeding together of the better adapted, resulting from 
their supplanting their rivals without allowing them full oppor- 
tunity 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 gen- 
eration 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. As the breeding together of the more 
vigorous and the better adapted, caused by their superior success, 
tends to increase and intensify the vigor 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 propor- 
tion of offspring produced by the more fertile, tends to increase the 
fertility of successive generations. Among those that would be equally 
productive if equally nourished, the ratio of propagation varies directly 
as the degree of sustentation above a certain minimum (and perhaps 
below a certain maximum), and therefore directly as the degree of 
adaptation that secures this sustentation. This propagation according 
to degrees of adaptation to the environment is what I understand by natural 
selection. But among those that are equally adapted to the environ- 
ment the ratio of propagation varies directly as the ratio of fertility. 
This propagation according to degrees of fertility is what I call the law 
of cumulative fertility. It is not due to different degrees of success, or 
to any advantage which the individuals of one form have over those of 
other forms; but simply to the higher ratio of multiplication in the 
more fertile forms securing the inter-generation of the more fertile. Jn 
connection with natural selection it insures, in the descendants, the predomi- 
nance of the better adapted of the more fertile and the more fertile of the 
better adapted. 
At the close of the previous chapter I called attention to the fact 
that innumerable local segregations and other imperfect forms of 
se-generation are being constantly broken down, partly by the increase 
of numbers and partly by the superior fertility and vigor of offspring 
produced by crossing. It seems to be a fundamental law that vigor 
and variation in the offspring depend on some degree of diversity of 
constitution in the parents, and diversity of constitution that is not 
