IA8 -STUDY OF NATURAL HISTORY. 
generally few, and are usually confined to two or, 
at the most, three particulars; sometimes, indeed, 
where the group is much diversified, to only one. 
Thus the Sylvicole, already alluded to (167.), may be 
distinguished among the warblers solely by the form 
of their wings: but, if we wish to define them more 
decidedly, and to detach them from all other birds, 
we must, in addition to their own peculiar charac- 
ter, add that of the family to which they belong, 
namely, the warblers: we thus get the union already 
spoken of. By one we separate the Sylvicole from 
all other birds excepting the warblers; and by the 
other we point out those peculiarities which make 
them a particular division of warblers. This mode 
of definition is equally applicable to every group in 
nature, from the highest to the lowest. Where we 
can meet with three strongly marked characters, 
they may safely be employed; but one or, perhaps, 
two of them will always be found less universal than 
the other. When we come to the confines of a group 
so distinguished, the characters laid down for it 
gradually disappear, until at length only one out of 
three will be detected ; that, therefore, which is most 
universal, is the most essential. 
(172.) By simplicity of definition is meant, not a 
mere form of words, however desirable that may be, 
but the employment of such characters only as are 
necessary to the determination of the group, or 
object, in question. Thus, the family of hornbills 
or of fowcans,—the one known by protuberances on 
the bills, the other by the excessive size and smooth- 
ness of theirs, —are sufficiently detached from all 
other birds, simply by these circumstances. We 
