38 CATEGORIES OF CLASSIFICATION. 



character of the structure, which remains the 

 same in all, being, in fact, an articulated cylin- 

 der with variously combined rings and more or 

 less complicated tubular appendages. 



Branches or types, then, are natural groups 

 of the animal kingdom, founded on plans of 

 structure or structural ideas. What now are 

 classes ? Are they lesser divisions, differing only 

 in extent, or are they founded on special charac- 

 ters ? I believe the latter view to be the true 

 one, and that class characters have a signifi- 

 cance quite different from that of their mere 

 range or extent. These divisions are founded 

 on certain categories of structure ; and were 

 there but one animal of a class in the world, 

 if it had those characters on which a class is 

 founded, it would be as distinct from all other 

 classes as if its kind were counted by thousands. 



Baer approached the idea of the classes when 

 he discriminated between plan of structure or 

 type and the degree of perfection in the struc- 

 ture. But while he understands the distinction 

 between a plan and its execution, his ideas re- 

 specting the different features of structure are 

 not quite so precise. He does not, for instance, 

 distinguish between the complication of a given 

 structure and the mode of execution of a plan, 

 both of which are combined in what he calls 

 degrees of perfection. And yet, without this 



