VII 
ON THE INFERTILITY OF CROSSES 
173 
a fact of great importance in considering the origin of species 
bj' natural selection, since it shows us that, so soon as a slight 
differentiation of form or colour has been effected, isolation 
will at once arise by the selective association of the animals 
themselves; and thus the great stumbling-block of “the 
swamping effects of intercrossing,” which has been so pro¬ 
minently brought forward by many naturalists, will be com¬ 
pletely obviated. 
If now we combine with this fact the correlation of colour 
with important constitutional peculiarities, and, in some cases, 
with infertility; and consider, further, the curious parallelism 
that has been shown to exist between the effects of changed 
conditions and the intercrossing of varieties in producing 
either an increase or a decrease of fertility, we shall have 
obtained, at all events, a starting-point for the production of 
that infertility which is so characteristic a feature of distinct 
species when intercrossed. All we need, now, is some means 
of increasing or accumulating this initial tendency; and to a 
discussion of this problem we will therefore address ourselves. 
The Influence of Natural Selection upon Sterility and Fertility. 
It will occur to many persons that, as the infertility or 
sterility of incipient species would be useful to them when 
occupying the same or adjacent areas, by neutralising the 
effects of intercrossing, this infertility might have been in¬ 
creased by the action of natural selection ; and this will be 
thought the more probable if we admit, as we have seen 
reason to do, that variations in fertility occur, perhaps as 
frequently as other variations. Mr. Darwin tells us that, at 
one time, this appeared to him probable, but he found the 
problem to be one of extreme complexity; and he was also 
influenced against the view by many considerations which 
seemed to render such an origin of the sterility or infertility 
of species when intercrossed very improbable. The fact that 
species which occupy distinct areas, and which nowhere come 
in contact with each other, are often sterile when crossed, is one 
of the difficulties; but this may perhaps be overcome by the 
consideration that, though now isolated, they may, and often 
must, have been in contact at their origination. More 
important is the objection that natural selection conld not 
