CLASSIFICATION OF THE MAMMALIA 53 



themselves, provided that they are all connected by inter- 

 gradations, and the more or less constant varieties or sub- 

 species are to be distinguished from the individual variants, 

 which are inconstant and fluctuating. No two specimens 

 agree exactly in every particular, but if a very large suite of 

 them be compared, it will be found that the great majority 

 depart but little from the average or norm of the species, and 

 the wider the departure from the norm, the fewer the indi- 

 viduals which are so aberrant. Taking so easily measured a 

 character as size, for example, and measuring several hundred 

 or a thousand representatives of some species, we see that a 

 large majority are of average size, a little more or a little less, 

 while very large or very small individuals are rare in propor- 

 tion to the amount by which they exceed or fall short of the 

 norm. Subspecies or varieties are marked by differences 

 which are relatively constant, but not of sufficient importance 

 to entitle them to rank as species. 



A group of the second rank is called a genus, which may 

 contain few or many species, or only a single one. In the latter 

 case the species is so isolated in character that it cannot prop- 

 erly be included in the same genus with any other species. 

 A large genus, one containing numerous species, is frequently 

 divisible into several subgenera, each comprising a group of 

 species which are more similar to one another than they are 

 to the other species of the genus. 



The third of the main groups in ascending order is the 

 family, which ordinarily consists of a number of genera united 

 by the possession of certain common characters, which, at 

 the same time, distinguish them from other genera, though 

 a single isolated genus may require a separate family for its 

 reception. Just as it is often convenient to divide a genus 

 into subgenera, so families containing many genera are usually 

 divisible into subfamilies, as indicative of closer relationships 

 within the family. The name of the family is formed from 

 that of the genus first described or best known, with the 



