102 



INTRODUCTION TO ZOOLOGY. 



of many degrees of complexity. As we have yet 

 studied only one branch, we sliall take the inverte- 

 brates as a whole and let them be repi-esented by a 

 worm. 



The simplest class of vertebrates is the fish. The 

 next group of vertebrates, representing a greater 

 degree of complexity, is the group which includes 

 batrachians, reptiles, and birds. We may take the 

 reptiles as representing this group. 



The animals of greatest complexity are repre- 

 sented by the mammals, of which we take the rabbit 

 as a type. 



Arranging these living animals in a series, we shall 

 have the following : — 



Table of Animals Kow Lilving-. 



This is called the Zoologic or Taxonomic Series. 



Next, if we take the animals that have existed and 

 arrange them in the order of their appearance upon 

 the earth, we shall find that the first animals which 

 appeared were undoubtedly protozoans. Each one 

 consisted of a single animal cell. Therefore, they 

 were comparable in complexity to an egg. 



The next group of animals to appear were inverte- 

 brates. One great period of time is called the Age of 

 Invertebrates. We may let the worm represent the 

 animals which appeared at that time. 



Another geologic period is called the Age of Fishes, 

 which succeeded immediately the Age of Invertebrates. 



Then followed the Age of Reptiles, and finally the 

 Age of Mammals. 



