222 THE DESCENT OF MAN. Part I. 



there is in hybrids between lessened fertility and vitality : 

 other analogous cases could be added. 



Even if it should hereafter be proved that all the 

 races of men were perfectly fertile together, he 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 fertility 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 habits 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 favour of the Pallasian 

 doctrine 13 that domestication tends to eliminate the 



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 inca- 

 pacity of certain trees to be grafted together, is incidental on other 

 acquired differences. 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 element 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 repro- 

 ductive system, and we have good reason to believe (as before re- 

 marked) that the fluctuating conditions of domestication tend to elimi- 

 nate that sterility which is so general with species in a natural state 

 when crossed. It has elsewhere been shewn by me (ibid. vol. ii. p. 185, 

 and' ' Origin of Species,' oth 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 



