DARWINISM 
CHAP. 
17(5 
greatly assisted by two other important factors in the 
problem. It has already been shown that, with each 
modification of form and habits, and especially with modifica¬ 
tions of colour, there arises a disinclination of the two forms 
to pair together; and this would produce an amount of 
isolation which would greatly assist the specialisation of the 
forms in adaptation to their different conditions of life. 
Again, evidence has been adduced that change of conditions 
or of mode of life is a potent cause of disturbance of the 
reproductive system, and, consequently, of infertility. We 
may therefore assume that, as the two forms adopted more 
and more different modes of life, and perhaps acquired also 
decided peculiarities of form and coloration, the infertility 
between them would increase or become more general; and as 
we have seen that every such increase of infertility would 
give that portion of the species in which it arose an advantage 
over the remaining portions in which the two varieties were 
more fertile together, all this induced infertility would main¬ 
tain itself, and still further increase the general infertility be¬ 
tween the two forms of the species. 
It follows, then, that specialisation to separate conditions 
of life, differentiation of external characters, disinclination to 
cross-unions, and the infertility of the hybrid produce of these 
unions, would all proceed pari passu, and would ultimately 
lead to the production of two distinct forms having all the 
characteristics, physiological as well as structural, of true 
species. 
In the case now discussed it has been supposed, that some 
amount of general infertility might arise in correlation with 
the different modes of life of two varieties or incipient 
species. A considerable body of facts already adduced 
renders it probable that this is the mode in which any 
widespread infertility would arise; and, if so, it has been 
shown that, by the influence of natural selection and the 
known laws which affect varieties, the infertility would be 
gradually increased. But, if we suppose the infertility to 
arise sporadically within the two forms, and to affect only a 
small proportion of the individuals in any area, it will be 
difficult, if not impossible, to show that such infertility would 
have any tendency to increase, or would produce any but a 
