248 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 Sylvicolce, already alluded to (167«), maybe 

 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 

 sooken of. By one we separate the Sylvicolce 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 toucans, — 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 



