124 THE LIVING WORLD. 



eral we may say that, at the beginning of their history, 

 all groups of animals are few in numbers and of 

 slight variety ; then they expand until a culmination 

 is reached, when they begin to diminish in numbers 

 until they reach extinction, and are replaced by 

 other groups. Such seems to be the general history 

 of all groups. Taken separately, however, the groups 



Q 



Te 



Cr 



Jr 



Tr 

 P 

 C 



D 



FIG. 15. 



differ widely from each other, some of them having 

 become extinct long ago, and others not yet seeming 

 to have reached their culmination. Although the 

 history of the different orders and classes is almost 

 as varied as the groups themselves, still we may 

 recognize several distinct types of development. 



Fig. 1 5. Some groups of animals were in existence 

 in abundance at the beginning of our record ; they 



