322 Agassiz’s Contributions to the 
“Upon the closest scrutiny of the subject, I find that these divisions 
cover all the categories of relationship which exist among animals, as far 
as their structure is concerned. 
“ Branches or types are characterized by the plan of their structure, 
“ Classes, by the manner in which that plan is executed, as far as ways 
and means are concerne 
edged in a natural zodlogical system; but these are not to be traced 80 
uniformly in all classes as the former,—they are in reality only limita 
” 
tions of the other kinds of divisions. 
1. Branches.—The Branches correspond to the ideal plans of 
structure in the Animal Kingdom, without any reference to the 
mode of expressing that plan in form or structure. They are 
the four Archetypes, recognized by Cuvier, the Radiate, Mol- 
luscan, Articulate, and the Vertebrate: the idea in the first, 4 
radiate arrangement in the interior structure whatever that struc 
t 
ure, the second and others having a bilateral symmetry ; in ae 
they are founded upon distinct plans of structure, ties 
distinct moulds or forms. Now there can certainly be no reason WHY’ 
* It is almost superfluous for me to mention here that the terms plan, 
mean er in which a plan is carried out, complication flan ly used in 
details of structure, ultimate structure, relations of individuals, frequent eh meal 
the following pages, are taken in a somewhat different sense from their us' 
, as is always necessary when new views are introduced ina ger | 
seize at once upon the form best adapted to carry convic’ ex 
i views more immediately with on 
all 
