50 Darwin, and after Darwin. 



modification has been in both cases strictly analogous 

 Why then this conspicuous difference with respect to 

 the reproductive system ? 



The answer is simple. It has never been the object 

 of breeders or of horticulturists to select variations 

 in the direction of cross-infertility, for the swamping 

 effects of intercrossing are much more easily and 

 rapidly prevented by artificial isolation. Consequently, 

 although they have been able to modify natural types 

 in so many directions and in such high degrees with 

 regard to morphology, there has been no accompanying 

 physiological modification of the kind required. But 

 in nature there is no such thing as artificial, i.e. in- 

 tentional, isolation. Consequently, on common areas 

 it must usually happen that those changes of mor- 

 phology which are associated with cross-infertility 

 are the only ones which can arise. Hence the very 

 remarkable contrast between our domesticated varie- 

 ties and natural species with regard to cross-infertility 

 is just what the present theory would expect, or, 

 indeed, require. But on any other theory it has 

 hitherto remained inexplicable. 



In particular, the contrast in question has consti- 

 tuted one of the main difficulties with which the theory 

 of natural selection has hitherto had to contend, not 

 only in the popular mind, but also in the judgement 

 of naturalists, including the joint-authors of the theory 

 themselves. Thus Darwin says : 



The fertility of varieties is, with reference to my theory, 

 of equal importance with the sterility of species, for it seems 

 to make a broad and clear distinction between varieties and 

 species 1 . 



1 Origin of Species, p. 236. 



