II THE ORIGIN OF SPECIES 27 



from the former point of view, a species is nothing 

 more than a kind of animal or plant, which is 

 distinctly definable from all others, by certain 

 constant, and not merely sexual, morphological 

 peculiarities. Thus horses form a species, because 

 the group of animals to which that name is applied 

 is distinguished from all others in the world by 

 the following constantly associated characters. 

 They have 1, A vertebral column ; 2, Mammae ; 

 3, A placental embryo ; 4, Four legs ; 5, A single 

 well-developed toe in each foot provided with a 

 hoof; 6, A bushy tail; and 7, Callosities on the 

 inner sides of both the fore and the hind legs. 

 The asses, again, form a distinct species, because, 

 with the same characters, as far as the fifth in the 

 above list, all asses have tufted tails, and have 

 callosities only on the inner side of the fore-legs. 

 If animals were discovered having the general 

 characters of the horse, but sometimes with cal- 

 losities only on the fore-legs, and more or less 

 tufted tails ; or animals having the general char- 

 acters of the ass, but with more or less bushy 

 tails, and sometimes with callosities on both pairs 

 of legs, besides being intermediate in other re- 

 spects the two species would have to be merged 

 into one. They could no longer be regarded as 

 morphologically distinct species, for they would 

 not be distinctly definable one from the other. 



However bare and simple this definition of 

 species may appear to be, we confidently appeal to 



