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. 

 During 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 species 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, 

 wall 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 



