84 THE EELATIONS OF MAN 



Reptiles, Birds, and Mammals, and tliese into smaller 

 groups called ' Orders ; ' these into ' Families ' and ' Gen- 

 era ;' while the last are finally broken up into the smallest 

 assemblages, which are distinguished by the possession of 

 constant, not-sexual, characters. These ultimate groups 

 are Species. 



Every year tends to bring about a greater uniformity 

 of opinion throughout the zoological world as to the limits 

 and characters of these groups, great and small. At pres- 

 ent, for example, no one has the least doubt regarding the 

 characters of the classes Mammalia, Aves, or Reptilia ; 

 nor does the question arise whether any thoroughly 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. 



Ko one doubts, for example, that the Sloth and the 

 Ant-eater, the Kangaroo and the Opossum, the Tiger and 

 the Badger, the Tapir and the Rhinoceros, are respect- 

 ively 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 

 vertebrse ; the adaptation of their frames to climbing, 

 leaping, or running ; the number and form of their teeth ; 

 and the characters of their skulls and of the contained 

 brain. But, with all these differences, they are so closely 

 connected in all the more important and fundamental 

 characters of their organization, and so distinctly sepa- 

 rated by these same characters from other animals, that 

 zoologists find it necessary to group them together as 

 members of one order. And if any new animal were dis- 

 covered, and were found to present no greater difference 



