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 
