4 ANATOMY OF THE RABBIT 



With succession has come also modification, as is shown by the 

 appearance on the earth at different geological times of progressive- 

 ly more specialized animals, which reveal in a large way the same 

 kinds of differences observable among primitive and specialized 

 animals 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 comparative 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 pouches 

 in the embryonic condition, though these are never used for func- 

 tional gills, is in itself an important fact bearing on adult structure, 

 but such a condition also illustrates how extensively 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 re- 

 semblances 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 re- 

 semblances 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 — or homologous. 

 On the other hand, their differences, particularly the differences in 

 homologous parts, are chiefly marks of divergence in evolution. It 

 is conceivable that many of the internal features of animals are 

 the result of a general progressive development brought about by 

 some inherent force in the constitution of the successive generations 

 of organisms themselves, more conspicuous in comparison of a 



