: 
Natural History of the United States. 323 
we should find practically that such groups may be traced in nature, 
fose who may not see them may deny their existence; those who r 
nize them may vary in their estimation of their natural limits; but all 
can, for the greatest benefit of science, agree to call any group which 
Seems to them to be founded upon a special plan of structure, a type or 
ion among naturalists respecting their limits, let the discussion upon this 
point be carried on with the understanding that types are to be charac- 
terized by different plans of structure, and not by special anatomical pe- 
culiarities. Let us avoid confounding the idea of plan with that of com- 
Senge of structure, even though Cuvier himself has made this mistake 
ere and there in his classification. 
“The best evidence I can produce that the idea of distinct plans of 
structure is the true pivot upon which the natural limitation of the 
branches of the animal kin dom is ultimately to turn, lies in the fact 
Reference to the plan adopted in framing it; secondly, 
2. Classes.—Under this head, Professor Agassiz remarks: 
“Structure may be considered from many points of ve Acne 
in Work to be done by it, and to the ways and means employed in 
gilding it up; thirdly, with reference to the degr 
plication which it exhibits, which may differ gre 
