94 MAN AND THE LOWER ANIMALS. n 



formity of opinion throughout the zoological 

 world as to the limits and characters of these 

 groups, great and small. At present, for example, 

 no one has the least doubt regarding the charac- 

 ters of the classes Mammalia, Aves, or Reptilia; 

 nor does the question arise whether any thor- 

 oughly well-known animal should be placed in 

 one class or the other. Again, there is a very 

 general agreement respecting the characters and 

 limits of the orders of Mammals, and as to the 

 animals which are structurally necessitated to 

 take a place in one or another order. 



No one doubts, for example, that the Sloth 

 and the Ant-eater, the Kangaroo and the Opos- 

 sum, the Tiger and the Badger, the Tapir and 

 the Ehinoceros, are respectively members of the 

 same orders. These successive pairs of animals 

 may, and some do, differ from one another im- 

 mensely, in such matters as the proportions and 

 structure of their limbs; the number of their 

 dorsal and lumbar vertebra?; the adaptation of 

 their frames to climbing, leaping, or running; the 

 number and form of their teeth; and the char- 

 acters of their skulls and of the contained brain. 

 But, with all these differences, they are so closely 

 connected in all the more important and funda- 

 mental characters of their organization, and so 

 distinctly separated by these same characters from 

 other animals, that zoologists find it necessary to 

 group them together as members of one order. 



