LEADING GROUPS OF EXISTING SYSTEMS 171 



SECTION VI 

 SPECIES 



It is generally believed that nothing is easier than to determine 

 species and that, of all the degrees of relationship which animals 

 exhibit, that which constitutes specific identity is the most clearly 

 defined. An unfailing criterion of specific identity is even supposed 

 to exist in the sexual connection which so naturally brings together 

 the individuals of the same species in the function of reproduction. 

 But I hold that this is a complete fallacy, or at least a petitio prin- 

 cipii, not admissible in a philosophical discussion of what truly 

 constitutes the characteristics of species. I am even satisfied that some 

 of the most perplexing problems involved in the consideration of 

 the natural limits of species would have been solved long ago, had 

 it not been so generally urged that the ability and natural disposition 

 of individuals to connect themselves in fertile sexual intercourse 

 was of itself sufficient evidence of their specific identity. Without 

 alluding to the fact that every new case of hybridity^* is an ever- 

 returning protest against such an assertion, and without entering 

 here into a discussion respecting the possibility or practicability 

 of setting aside this difficulty by introducing the consideration of 

 the limited fertility of the progeny of individuals of different species, 

 I will only remark that as long as it is not proved that all the varieties 

 of dogs and of any others of our domesticated animals, and of our 

 cultivated plants, are respectively derived from one unmixed species, 

 and as long as doubts can be entertained respecting the common 

 origin of all races of men from one common stock, it is not logical 

 to admit that sexual connection resulting even in fertile offspring is a 

 trustworthy evidence of specific identity. 



To justify this assertion, I would only ask, where is the unpreju- 

 diced naturalist who in our days would dare to maintain: 1st, that it 

 is proved that all the domesticated varieties of sheep, of goats, of 

 bulls, of llamas, of horses, of dogs, of fowls, etc., are respectively 



^*Braun, Betrachtungen iiber die Erscheinung der Verjiingung; Samuel George 

 Morton, "Hybridity in Animals . . . ," American Journal of Science, III (2d ser., 

 1847), 3&-50, 203-212. and "Additional Observations on Hybridity in Animals . . . ," 

 Charleston Medical Journal, V (1850), 755-805. 



