184 
DARWINISM 
CHAP. 
chance of survival in the battle of life. It is, therefore, 
directly under the control of natural selection, which acts 
both by the self-preservation of fertile and the self-destruction 
of infertile stocks — except -always where correlated as above, 
when they become useful, and therefore subject to be increased 
by natural selection. 
Summary and Concluding Remarks on Hybridity. 
The facts which are of the greatest importance to a com¬ 
prehension of this very difficult subject are those which show 
the extreme susceptibility of the reproductive system both in 
plants and animals. We have seen how both these classes of 
organisms may be rendered infertile, by a change of conditions 
which does not affect their general health, by captivity, or 
by too close interbreeding. We have seen, also, that infertility 
is frequently correlated with a difference of colour, or with other 
characters; that it is not proportionate to divergence of 
structure ; that it varies in reciprocal crosses between pairs of 
the same species ; while in the cases of dimorphic and tri- 
morphic plants the different crosses between the same pair 
of individuals may be fertile or sterile at the same time. It 
appears as if fertility depended on such a delicate adjustment 
of the male and female elements to each other, that, unless 
constantly kept up by the preservation of the most fertile 
individuals, sterility is always liable to arise. This preservation 
always occurs within the limits of each species, both because 
fertility is of the highest importance to the continuance of the 
race, and also because sterility (and to a less extent infertility) 
is self-destructive as well as injurious to the species. 
So long therefore as a species remains undivided, and in 
occupation of a continuous area, its fertility is kept up by 
natural selection ; but the moment it becomes separated, 
either by geographical or selective isolation, or by diversity 
of station or of habits, then, while each portion must be kept 
fertile inter se, there is nothing to prevent infertility arising 
between the two separated portions. As the two portions 
will necessarily exist under somewhat different conditions of 
life, and will usually have acquired some diversity of form and 
colour — both which circumstances we know to be either the 
cause of infertility or to be correlated with it, — the fact of 
