XIL] THE ORIGIN OF SPECIES. 225 



conception of the nature of the objects to which the word &quot; species &quot; 

 is applied ; but it has, perhaps, occurred to a few, even to those 

 who are naturalists ex professo, to reflect, that, as commonly 

 employed, the term has a double sense and denotes two very 

 different orders of relations. When we call a group of animals, 

 or of plants, a species, we may imply thereby, either that all 

 these animals or plants have some common peculiarity of form or 

 structure; or, we may mean that they possess some common 

 functional character. That part of biological science which deals 

 with form and structure is called Morphology that which 

 concerns itself with function, Physiology so that we may 

 conveniently speak of these two senses, or aspects, of &quot; species &quot; 

 the one as morphological, the other as physiological. Regarded 

 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 

 callosities only on the fore-legs, and more or less tufted tails ; or 

 animals having the general characters of the ass, but with more 

 or less bushy tails, and sometimes with callosities on both pairs of 

 legs, besides being intermediate in other respects 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 



Q 



