280 



AN INTRODUCTION TO ZOOLOGY 



order of their succession, and the time of origin and relative 

 number of the different groups of animals (279). 



TABLE XV 



THE GEOLOGICAL PERIODS AND THE ORIGIN AND RELATIVE NUMBER OF 

 THE DIFFERENT GROUPS OF ANIMALS 



ERA 



PERIOD 



i 



2 



E * 



s S < 



Cenozoic 



Recent 



Pleistocene 



Pliocene 



Miocene 



Eocene 



r 10 



3,000,000 



Mesozoic 



Cretaceous 



Jurassic 



Triassic 



7,200,000 



Paleozoic 



Permian 



Carboniferous 



Devonian 



Silurian 



Cambrian 



> f 17,500,000 



Archaean 



Laurentian 



(3) Variations in Animals 



We have already noted (p. 79) that no two organisms are ever 

 exactly alike. In other words, individuals of different species 

 are so dissimilar as to be in most cases easily recognized. Indi- 



