A GENERAL REVIEW OF THE ANIMAL KINGDOM 143 



the preceding. This is partly because the animals composing 

 it differ more among themselves than in the other classes. It 

 includes snakes, lizards, turtles, and crocodiles. The reptiles 



FIG. 66. 



REPTILES 



5500 



POaiFERA 



2500 



FIG. 66. Diagram showing the general relations of the chief divisions of the animal kingdom. 

 The number of species belonging to each is roughly approximate, only. During the last 30 years 

 the number of species of animals recognized by zoologists has increased at the average rate of 

 12,000 per year. 



have some features which indicate that they may be distantly 

 related to both birds and mammals, as well as to the next 

 class. This is an additional reason why the group of reptiles 

 is a difficult one to define. In general they may be recognized 



