12 Anatomy of the Rabbit. 



With succession has also come modification, the evidence of 

 which lies not only, geologically speaking, in the relative times of 

 appearance of life forms on the earth, but also in the fact that 

 succession leads from primitive to specialized animals, revealing 

 in a large way the same kinds of differences observable among those 

 living at the present day. That the entire skeleton of a mammal 

 is patterned upon the primitive skeleton of the fossil amphibia 

 of the Carboniferous and Permian is evident from a comparison of 

 the components part for part, but it is equally evident from com- 

 parative anatomy that the viviparous condition of a higher mammal 

 is founded upon an oviparous condition in lower forms, even if no 

 fossil evidence is forthcoming. That a mammal as an air-breathing 

 vertebrate should develop gill structures in the embryonic con- 

 dition, though circumstances never come about by which such 

 structures are used, is in itself an important fact bearing on adult 

 structure, but such a condition also shows to what extent a living 

 animal carries ancestral features, whether functionally modified 

 or not. 



All characters of animals have thus an evolutionary basis, the 

 general nature of which is easily understood, although the process 

 by which they have been developed is still a matter of uncertainty. 

 In comparison with one another, animals present certain resem- 

 blances and differences — diagnostic features, which are used as a 

 basis for classifying them into major and minor groups. In many 

 cases characters of resemblance have been shown to be secondary, 

 and are hence described as convergent. In some of these the 

 resemblances are of a gross type, and the structures are described 

 as analogous ; in other cases they are exact or homoplastic. 

 As a rule, however, characters of resemblance are broad marks of 

 affinity, comparable to those seen on a small scale in human 

 families, or in human races, and determined as in the latter cases 

 by heredity. The chief basis of comparison of animals with one 

 another is the general assumption that structures which are similar 

 or identical are homogenous- of common origin On the other 

 hand, their differences are chiefly marks of divergence in evolution. 

 Although it is conceivable that many of the internal features of 

 animals are the result of a general progressive development, more 



