VII 
ON THE INFERTILITY OF CROSSES 
185 
some degree of infertility usually appearing between closely 
allied but locally or physiologically segregated species is exactly 
what we should expect. 
The reason why varieties do not usually exhibit a similar 
amount of infertility is not difficult to explain. The popular 
conclusions on this matter have been drawn chiefly from what 
occurs among domestic animals, and we have seen that the 
very first essential to their becoming domesticated was that 
they should continue fertile under changed conditions of life. 
I luring the slow process of the formation of new varieties by 
conscious or unconscious selection, fertility has always been 
an essential character, and has thus been invariably preserved 
or increased; while there is some evidence to show that 
domestication itself tends to increase fertility. 
Among plants, wild species and varieties have been more 
frequently experimented on than among animals, and we 
accordingly find numerous cases in which distinct species of 
plants are perfectly fertile when crossed, their hybrid offspring 
being also fertile inter se. We also find some few examples of 
the converse fact—varieties of the same sjiecies which when 
crossed are infertile or even sterile. 
The idea that either infertility or geographical isolation is 
absolutely essential to the formation of new species, in order 
to prevent the swamping effects of intercrossing, has been 
shown to be unsound, because the varieties or incipient 
species will, in most cases, be sufficiently isolated by 
having adopted different habits or by frequenting different 
stations; while selective association, which is known to be 
general among distinct varieties or breeds of the same species, 
will produce an effective isolation even when the two forms 
occupy the same area. 
From the various considerations now adverted to, Mr. 
Darwin arrived at the conclusion that the sterility or in¬ 
fertility of species with each other, whether manifested in the 
difficulty of obtaining first crosses between them or in the 
sterility of the hybrids thus obtained, is not a constant or 
necessary result of specific difference, but is incidental on 
unknown peculiarities of the reproductive system. These 
peculiarities constantly tend to arise under changed conditions 
owing to the extreme susceptibility of that system, and they 
