214 THE DESCENT OF MAN. [Part I. 



of men were perfectly fertile together, lie who was inclined 

 from other reasons to rank them as distinct species, might 

 with justice argue that fertility and sterility are not safe 

 criterions of specific distinctness. We know that these 

 qualities are easily affected by changed conditions of life 

 or by close inter-breeding, and that they are governed by 

 highly complex laws, for instance that of the unequal fer- 

 tility of reciprocal crosses between the same two species. 

 With forms which must be ranked as undoubted species, 

 a perfect series exists from those which are absolutely 

 sterile when crossed, to those which are almost or quite 

 fertile. The degrees of sterility do not coincide strictly 

 with the degrees of difference in external structure or hab- 

 its of life. Man in many respects may be compared with 

 those animals which have long been domesticated, and a 

 large body of evidence can be advanced in favor of the 

 Pallasian doctrine, 13 that domestication tends to eliminate 



13 ' The Variation of Animals and Plants under Domestication,' vol. ii. 

 p. 109. I may here remind the reader that the sterility of species when 

 crossed is not a specially-acquired quality ; but, like the incapacity of 

 certain trees to be grafted together, is incidental on other acquired dif- 

 ferences. The nature of these differences is unknown, but they relate 

 more especially to the reproductive system, and much less to external 

 structure or to ordinary differences in constitution. \ One important ele- 

 ment in the sterility of crossed species apparently lies in one or both 

 having been long habituated to fixed conditions ; for we know that 

 .changed conditions have a special influence on the reproductive system, 

 and we have good reason to believe (as before remarked) that the fluctu- 

 ating conditions of domestication tend to eliminate that sterility which is 

 so general with species in a natural state when crossed. It has elsewhere 

 been shown by me (ibid. vol. ii. p. 185, and ' Origin of Species,' 5th edit, 

 p. 317) that the sterility of crossed species has not been acquired through 

 natural selection : we can see that when two forms have already been 

 rendered very sterile, it is scarcely possible that their sterility should be 

 augmented by the preservation or survival of the more and more sterile 

 individuals ; for as the sterility increases fewer and fewer offspring will 

 be produced from which to breed, and at last only single individuals will 

 be produced, at the rarest intervals But there is even a higher 



