210 STUDY OF NATURAL HISTORY. 
show us is absolutely essential to the harmony of the 
whole. 
(145.) The student must not, however, suppose 
that all sub-genera are so comprehensive, or so 
readily detected, as those which, for the sake of 
strong examples, we have just instanced: very many, 
so far as we yet know, are composed but of one 
species; and, generally speaking, the number of 
species is small. He must not, therefore, be ap- 
prehensive he is carrying the above theory too far, 
when, in the arrangement of his collection, he places 
at short intervals of separation, many insects, as 
probable types of sub-genera, of which he has 
but one example. His fears, that he is making 
needless divisions, may be quieted by two con-. 
siderations: first, that natural groups do not depend 
on their numerical amount of species; and, secondly, 
from the amazing number of nature’s productions 
already known, and of which he has not, probably, 
seen one tenth part of such as actually exist in 
collections, setting aside those which have not yet 
been discovered. Even admitting that his collection 
is very extensive, and that there is consequently a 
greater chance of his finding more than a solitary 
example of a supposed sub-genus, still he will fre- 
quently be deceived in his estimation of its extent, 
which can often only be learned from books, or from 
an extensive acquaintance with the contents of other 
cabinets. A singular and striking instance of this is 
afforded by the sub-genus Ludamus, first defined by 
us (Zool. Ill. 2. pl. 41.), and composed entirely of 
the swallow-tailed skipper butterflies of the family 
Hesperide. Only one species was known to Linnzeus; 
