SIMPLICITY OF DEFINITION. 249 



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 a monograph, or complete 

 account of the insect, every one of its characters 

 should of course be described. 



(173.) To attain this simplicity, however, 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 

 in the 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, 



