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 Eudamus, first defined by 

 us (Zool. III. 2. pi. 41.), and composed entirely of 

 the swallow-tailed skipper butterflies of the family 

 Hesperida. Only one species was known to Linnaeus ; 



