42 THE ORIGIN OF SPECIES 



highest degree improbable that similar conditions long- 

 continued should likewise induce this tendency: though 

 in certain cases, with species having a peculiar constitu- 

 tion, 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 existence with numer- 

 ous competitors, will have been exposed during long 

 periods of time to more uniform conditions than have 

 domestic varieties; and this may well make a wide differ- 

 ence in the result. For we know how commonly wild 

 animals and plants, when taken from their natural condi- 

 tions 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 manner be eminently sensitive to the influence of an 

 unnatural cross. Domesticated productions, on the other 

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

 tication, were not originally highly sensitive to changes 

 in their conditions of life, and which can now generally 

 resist with undiminished fertility repeated changes of con- 



