52 CLASSIFICATION. 



Every species therefore is considered to present a 

 certain combination of characters, which 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 nature 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 their 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- 



