TRANSFORMISM 209 



the human sense, that is, all the individuals compos- 

 ing it are connected together by a tie of common 

 descent. 



Let us extend this reasoning to categories of organ- 

 isms of higher orders than species. We can associate 

 together groups of species in the same way that we 

 associate together the individuals of the same species. 

 There are certain morphological characters which are 

 common to all the species in the category, but there 

 are also differences between specific group and specific 

 group, and these differences may be regarded as 

 variations from the generic morphological type. All 

 the Cats, for instance, have certain characters in 

 common : fully retractile claws, a certain kind of 

 dentition, certain cranial characters, and so on. We 

 postulate a feline type of structure, and we then 

 regard the characters displayed by the cat, lion, tiger, 

 leopard, etc., as deviations from this feline m'oi-pho- 

 logical type. Thus we establish the Family Felidae. 

 But again we find that the Felidce together with the 

 Canidae, and many other species of animals, also display 

 common characters, dental and osteological chiefly, 

 and we express this resemblance by assembling all 

 these families in one Order, the Carnivora. The 

 Carnivores, however, are only one large group of 

 Quadrupeds : there are many others, such as the 

 Rodents, Ungulates, Cetacea, etc., and all of these 

 possess common characters. In all of tliem the 

 integument is provided with hairs, or other similarly 

 developed structures ; all breathe by means of a dia- 

 phragm; in all, the young are nourished by suckling 

 the mammae of the mother; and all develop on a 

 placenta. We therefore group them all in the Class 

 Mammaha. Now the Mammals possess an internal 

 skeleton of which the most fundamental part is an 



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