250 STUDY OF NATURAL HISTORY. 
having ascertained such a fact as this, through an 
indefinite number of groups, our next business is to 
see how far it can be traced in groups of larger di- 
mensions; and finally, whether it is prevalent in 
quadrupeds, birds, and other vertebrated animals. 
Should we succeed in this, we obviously demon- 
strate that, through all her variations, nature has 
preserved, at least in one instance, a definite plan of 
variation, consisting in this, that in every natural 
group she gives to one of its types a preponderance 
of tail, or caudal appendages representing a tail. 
We maintain not, here, that such is actually the fact ; 
we are merely stating a case, to illustrate the mode 
of generalising the variation of characters just re- 
commended, and thereby simplifying the diagnosis 
by which the different forms in nature are to be 
distinguished. That this species of generalisation is 
not impracticable, at least in ornithology, has been 
clearly domonstrated in numerous groups defined on 
these principles in the Northern Zoology. And it 
follows, as a matter of induction, that if the vari- 
ations of one extensive class of animals are regulated 
by certain general laws, manifest in all the groups 
of that class, the same will be discovered in other 
portions of nature, so soon as they have been investi- 
gated with sufficient attention to such circumstances. 
(174.)-Yet, although no general rules will here 
be laid down for the discovery or selection of 
essential characters, experience has shown that they 
may be derived with more chance of success from 
- some circumstances than from others. It becomes 
desirable, therefore, to state what these circum- 
stances are, and to trace the influence they possess 
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