52 - CLASSIFICATION. 



Every species therefore is considered to present a 

 certain combination of characters_, wliicli occurs in 

 no other species^ but at the same time we find that 

 many of them may be common to several species, 

 some of these to a still larger number_, and so on, 

 until we obtain a series of groups gradually increasing 

 in comprehensiveness, as the peculiar characters of the 

 species from which we started are dropped one by 

 one; and when this process has been repeated with 

 all the species of the department of natm^e under 

 investigation, its classification is effected. 



To facilitate the mention and comparison of the 

 groups thus formed, each of them is provided with a 

 name, whilst theii' different values in the system are 

 indicated by means of particular terms applied uni- 

 formly to all groups of the same importance. Thus 

 the group formed by the immediate union of a certain 

 number of closely-allied species is denominated a 

 genus ; that produced by the combination of several 

 genera is called a family ; several families again, pos- 

 sessing common characters, may constitute a tribe, 

 and several closely-allied tribes an order. The com- 

 bination of orders forms a class ; but as Insects con- 

 stitute only a single class of animals, there is no 

 occasion for our pursuing this dry subject any further. 



It may be as well, however, to pay a little attention 

 to the principles on which names are applied to the 

 different groups above referred to, and especially to 

 the genera and species. The name given to a genus 

 usually indicates one of the more prominent cha- 

 racters possessed in common by all the species in- 

 cluded in it, so that, although it can never safely be 

 considered as giving the distinguishing peculiarity of 

 the genus, it very often serves as a useful memo- 



