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SIMPLICITY OF DEFINITION. 949 
need not, therefore, in giving essential characters, 
go on to describe other points of structure ; because 
they are not only unnecessary, but they distract the 
attention from those circumstances upon which it 
should be entirely fixed. So, in like manner, the 
genus Sylvicola needs only to be characterised by 
the bill and the wings ; all its other characters being 
common to the next group in rank, of which it forms 
a part. In the lepidopterous order of insects, the 
form of the wings will in almost all cases determine 
the sub-genus; although in amonograph, or complete 
account of the insect, every one of its characters 
should of course be described. 
(173.) To attain this simplicity, howéver, is much 
more difficult than would be at first imagined. For, 
as we find that no one peculiar set of organs can be 
universally employed for such distinctions; so it 
becomes necessary to discover, in the multiplicity 
of characters which every group presents, what is 
that one which is its peculiar and exclusive distinc- 
tion. Now this, as we before remarked, can only 
result from extensive analysis, or by generalising the 
mode in which natural groups are seen to vary. It 
is remarkable, in every natural group of the diurnal 
Lepidoptera, whether the group be large or small, 
that there is one modification in which the lower 
wings are always more or less tailed. Numerous 
and very striking instances of this have been given 
inthe Zoological Illustrations, selected from families 
and genera so different in themselves, as completely 
to do away with the idea that these swallow-tailed 
butterflies have any real affinity to each other, 
however strongly they are related by analogy. Now, 
