36 FERTILITY OP VARIETIES [Chap. IX. 



tinned domestication be admitted, and it can hardly 

 be rejected, it becomes in the highest degree improbable 

 that similar conditions long-continued should likewise 

 induce this tendency; though in certain cases, with 

 species having a peculiar constitution, sterility might 

 occasionally be thus caused. Thus, as I believe, we 

 can understand why with domesticated animals varieties 

 have not been produced which are mutually sterile; and 

 why with plants only a few such cases, immediately to 

 be given, have been observed. 



The real difficulty in our present subject is not, as it 

 appears to me, why domestic varieties have not become 

 mutually infertile when crossed, but why this has so 

 generally occurred with natural varieties, as soon as they 

 have been permanently modified in a sufficient degree 

 to take rank as species. We are far from precisely 

 knowing the cause; nor is this surprising, seeing how 

 profoundly ignorant we are in regard to the normal 

 and abnormal action of the reproductive system. But 

 we can see that species, owing to their struggle for ex- 

 istence with numerous competitors, will have been ex- 

 posed during long periods of time to more uniform 

 conditions, than have domestic varieties; and this may 

 well make a wide difference in the result. For we 

 know how commonly wild animals and plants, when 

 taken from their natural conditions and subjected to 

 captivity, are rendered sterile; and the reproductive 

 functions of organic beings which have always lived 

 under natural conditions would probably in like man- 

 ner be eminently sensitive to the influence of an un- 

 natural cross. Domesticated productions, on the other 

 hand, which, as shown by the mere fact of their domesti- 

 cation, were not originally highly sensitive to changes 



