8 VERTEBRATES: MAMMALS. 



branch are characterized by higher intelligence than 

 those of any other. Vertebrates comprise five * classes, 

 Mammalia, Birds, Reptiles, Batrachians, and Fishes. 

 Many writer's group the ' Batrachians and Reptiles to- 

 gether, and give the latter name to the class, thus mak- 

 ing the classes only four in number. In all these 

 the fundamental idea of a Vertebrate is plainly mani- 

 fested, and the principal parts of the skeleton corre- 

 spond, part to part, head to head, spinal column to spinal 

 column, locomotive members to locomotive members, 

 the members in each case modified according to the 

 function to be performed, whether it be that of standing, 

 or grasping, or walking, or running, or leaping, or spring- 

 ing, or flying, or creeping, or swimming, but the general 

 plan always the same. Figures 2-12, where corre- 

 sponding parts are marked by the same letter, show 

 clearly that the anterior locomotive members of differ- 

 ent Vertebrates are expressions of one and the same fun- 

 damental idea. 



SECTION I. 



THE CLASS OF MAMMALIA, OR MAMMALS. 



THE Class of Mammalia comprises all Vertebrates 

 which bring forth their young alive, and nourish them 

 with milk from their own bodies. They are all furnished 

 with a solid skeleton, which is divided into well-defined 

 regions, as the head, trunk, and extremities ; the upper 

 jaw is fixed to the cranium, the lower formed of only two 

 pieces ; the teeth are enamelled, and the neck, with few 

 exceptions, has only seven vertebrae. The brain is com- 



* Agassiz, in his "Essay on Classification," recognizes eight classes in 

 the Branch of Vertebrates, Mammals, Birds, Reptiles, Amphibians, 

 Selachians, Ganoids, Fishes proper, and Myzontes. 



