THE GENERAL SHAPES OF ANIMALS. 



187 



itsmxis, and also into other not very dissimilar halves by a 

 plane cuttiag it transversely. If, as shown in Figs. 275 

 and 276, analogous sections be made of a superior Eeptile, the 

 divided parts differ more decidedly. When a Mammal and a 

 Bird are treated in the same way, as shown in 'Figs. 277, 

 278, and Figs. 279, 280, the parts marked off by the divid- 



S7f 



ing planes are unlike in far greater degrees. On considering 

 the mechanical converse between organisms of these several 

 types and their environments — on remembering that the 

 fish habitually moves through a homogeneous medium of 

 nearly the same specific gravity as itself, that the terrestrial 

 reptile either crawls on the surface or raises itself very in- 

 ooinpletely above it, that the more active mammal, having 



