72 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



mits us to expect. But the groups in which there is evi- 

 dence of such modifications during" geologically recorded 

 time, all belong to the most differentiated members of their 

 classes. Lower forms, coextensive in duration, exhibit no 

 sign of having undergone any notable modification. While 

 the former are mutable^ the latter are persistent types in rela- 

 tion to geological time. 



Leaving the debatable question of the nature of Eozobn 

 aside, the oldest fossiliferous rocks are the Cambrian. The 

 scanty fauna therein preserved consists of forms which are 

 neither Protozoa nor JPorifera, nor even appertain to the 

 lowest groups of their respective classes. There is no reason 

 to believe that it gives a just notion of the contemporaneous 

 fauna, nor is there any valid reason for the supposition that 

 it represents the forms of animal life which were the first to 

 make their appearance on our planet. 



