THE DESCENT OR OBIGIN OF MAN 229 



consideration from their lessened fertility, may perhaps be 

 advanced as a proof of the specific distinctness of the parent 

 races. No doubt both animal and vegetable hybrids, when 

 produced from extremely distinct species, are liable to pre- 

 mature death; but the parents of mulattoes cannot be put 

 under the category of extremely distinct species. The com- 

 mon Mule, so notorious for long life and vigor, and yet so 

 sterile, shows how little necessary connection there is in 

 hybrids between lessened fertility and vitality, other analo- 

 gous cases could be cited. 



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 fer t-ility fl.nrl ptftrility qrp nnt q-afa 



criterions of_specific distjnctnssg- We know that these 

 qualities are easily affected by changed conditions of life, 

 or by close interbreeding, and that they are governed by 

 highly complex laws; for instance, that of the unequal 

 fertility of converse 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 completely fer- 

 tile. The degrees of sterility do not coincide strictly with 

 the degrees of difference between the parents in external 

 structure or habits of life. Man in many respects may be 

 compared with those animals which have long been domesti- 

 cated, and a large body of evidence can be advanced in favor 

 of the Pallasian doctrine, " that domestication tends to elimi- 



■* "The Variation of Animals and Plants tinder 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 differences. The nature 

 «f these differences is unknown, but they relate more especially to the repro- 

 ductive system, and much less so to external structure or to ordinary differences 

 in constitution. One important element in the sterility of crossed species ap- 

 parently lies in one or both having been long habituated to fixed conditions; 

 for we know that changed conditions have a special influence on the reproduc- 

 tive system, and we have good reason to believe (as before remarked) that the 

 fluctuating conditions of domestication tend to eliminate that sterility which 

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



