248 
REV. J. T. aULICK ON DIVERGENT EVOLUTION 
successive generations. Among those that would be equally pro- 
ductive 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 tvhat I understand by natural selection. But among those 
that are equally adapted to the environment the ratio of propa- 
gation varies directly as the ratio of fertility. This propagation 
according to degrees of fertility is ivhat I call the Law of Cumu- 
lative 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 multipli- 
cation in the more fertile forms securing the intergeneration of 
the more fertile. In connection with natural selection it ensures , 
in the descendants, the predominance 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 Segeneration are being constantly broken down, partly 
by the increase of numbers and partly by the superior fertility 
and vigour of offspring produced by crossing. It seems to be a 
fundamental law that vigour and variation in the offspring depend 
on some degree of diversity of constitution in the parents, and 
diversity of constitution that is not entirely fluctuating depends 
on some degree of Positive Segregation ; therefore vigour and 
variation depend on the breaking-down of incipient Segrega- 
tions, and on the interfusion of the slightly divergent forms 
that had been partially segregated. But in the history of 
every race that is winning success by its vigour and varia- 
tion there is liable to come a time when some variety, inher- 
iting sufficient vigour to sustain itself, even if limited to the 
benefits of crossing with the individuals of the same variety, 
becomes partially Segregated. As we have already seen, Segre- 
gation, in so far as it depends on the qualities of the organism, 
tends ever to become more and more intense ; but, in the very 
nature of things, not only will the Segregation be for many 
generations only partial, but partial Segregation, though it may 
greatly delay the submerging of different groups in one common 
group, will never prevent that result being finally reached. 
Though the siphon that connects two tanks of water be ever so 
