VII 
ON THE INFERTILITY OF CROSSES 
177 
prejudicial effect. If, for example, five per cent of each 
form thus varied so as to be infertile with the other form, 
the result would be hardly perceptible, because the individuals 
which formed cross-unions and produced hybrids would con¬ 
stitute a very small portion of the wdiole species; and the 
hybrid offspring, being at a disadvantage in the struggle for 
existence and being themselves infertile, would soon die out, 
while the much more numerous fertile portion of the two 
forms would increase rapidly, and furnish a sufficient number 
of pure-bred offspring of each form to take the place of the 
somewhat inferior hybrids between them whenever the 
struggle for existence became severe. We must suppose that 
the normal fertile forms would transmit their fertility to their 
progeny, and the few infertile forms their infertility; but 
the latter would necessarily lose half their proper increase 
by the sterility of their hybrid offspring whenever they 
crossed with the other form, and when they bred with their 
own form the tendency to sterility would die out except in 
the very minute proportion of the five per cent (one-twentieth) 
that chance would lead to pair together. Under these 
circumstances the incipient sterility between the two forms 
would rapidly be eliminated, and could never rise much above 
the numbers which were produced by sporadic variation each 
year. 
It was, probably, by a consideration of some such case as 
this that Mr. Darwin came to the conclusion that infertility 
arising between incipient species could not be increased by 
natural selection; and this is the more likely, as he was 
always disposed to minimise both the frequency and the 
amount even of structural variations. 
We have yet to notice another mode of action of natural 
selection in favouring and perpetuating any infertility that 
may arise between two incipient species. If several distinct 
species are undergoing modification at the same time and in 
the same area, to adapt them to some new conditions that 
have arisen there, then any species in which the structural or 
colour differences that have arisen between it and its varieties 
or close allies were correlated with infertility of the crosses 
between them, would have an advantage over the corre¬ 
sponding varieties of other species in which there was no such 
N 
