VII 
ON THE INFERTILITY OF CROSSES 
175 
Now, let us suppose that a partial sterility of the hybrids 
between the two forms arises, in correlation with the different 
modes of life and the slight external or internal peculiarities 
that exist between them, both of which we have seen to be 
real causes of infertility. The result will be that, even if the 
hybrids between the two forms are still freely produced, these 
hybrids will not themselves increase so rapidly as the two 
pure forms; and as these latter are, by the terms of the 
problem, better suited to their conditions of life than are 
the hybrids between them, they will not only increase more 
rapidly, but will also tend to supplant the hybrids altogether 
whenever the struggle for existence becomes exceptionally 
severe. Thus, the more complete the sterility of the hybrids 
the more rapidly will they die out and leave the two parent 
forms pure. Hence it will follow that, if there is greater 
infertility between the two forms in one part of the area than 
the other, these forms will be kept more pure wherever 
this greater infertility prevails, will therefore have an 
advantage at each recurring period of severe struggle for 
existence, and will thus ultimately supplant the less infertile 
or completely fertile forms that may exist in other portions 
of the area. It thus appears that, in such a case as here 
supposed, natural selection would preserve those portions of 
the two breeds which were most infertile with each other, or 
whose hybrid offspring were most infertile; and would, 
therefore, if variations in fertility continued to arise, tend to 
increase that infertility. It must particularly be noted that 
this effect would result, not by the preservation of the 
infertile variations on account of their infertility, but by the 
inferiority of the hybrid offspring, both as being fewer in 
numbers, less able to continue their race, and less adapted to 
the conditions of existence than either of the pure forms. It 
is this inferiority of the hybrid offspring that is the essential 
point; and as the number of these hybrids will be per¬ 
manently less where the infertility is greatest, therefore those 
portions of the two forms in which infertility is greatest will 
have the advantage, and will ultimately survive in the struggle 
for existence. 
The differentiation of the two forms into distinct species, 
with the increase of infertility between them, would be 
