CHORDATA 



413 



the theory of evolution, which is described in a subsequent 

 chapter, the higher animals have been evolved from the lower 

 in the course of ages, but the groups existing today do not 

 form a linear series from the lowest to the highest. The rela- 

 tionship is better expressed by a treelike arrangement, and the 

 following diagram will illustrate the probable position of the 

 various larger groups : — 



Mammalia 



Aves 



Cephalochorda 



Arthropoda 



Reptilia 



I 

 Amphibia 



I 

 Pisces 



I 



Vertebrata 



Chordata 



Vermes 



I 



Coelenterata 



Protozoa 



Enteropneusta 

 Tunicata 



Mollusca 



Echinodermata 



