250 PRINCIPLES OF ANIMAL BIOLOGY 



decides, in case the genus is later divided into two genera, which group 

 shall receive the original name. One genus constitutes the type of the 

 subfamily (when a subfamily exists), and one genus forms the type of 

 the family. The type is indicated by the describer or, if not indicated 

 by him, is fixed by another author. No two genera in the whole animal 

 kingdom may have the same name — a rule still occasionally violated 

 because the interested taxonomists have not proposed corrected names. 

 The name of a subfamily is formed by adding the ending -inae and the 

 name of a family by adding -idae to the root of the name of the type 

 genus. For example, Colubrinae and Colubridae are the subfamily and 

 family of snakes of which Coluber is the type genus. Names of sub- 

 families are accented on next to the last syllable, family names on the 

 third syllable from the end. 



The Basis of Classification. — Early systematists largely employed 

 superficial characters to differentiate and classify animals, and their 

 classifications were thus largely artificial and served principally as con- 

 venient methods of arrangement, description, and cataloguing. Since 

 the time of the development of the theory of descent with modifications 

 by Lamarck (1809) and Darwin (1859), as stated in an earlier section, 

 there has been an attempt to base the classification on relationships. 

 Very nearly related animals are put into the same species. They are 

 related because they descend from a common ancestry. The common 

 ancestry could not in most cases have been very ancient, otherwise 

 evolution within the group would have occurred and the species would 

 have been split into two or more species. Species that are much alike are 

 included in one genus, being thus marked off from the species of another 

 genus. The similarity of the species of a genus is held to indicate kin- 

 ship ; but since there is greater diversity among the individuals of a genus 

 than among the members of a species, the common stock from which the 

 species of a genus have sprung must have existed at an earlier time, in 

 order that evolution could bring about the degree of divergence now 

 observed. In like manner, a family is made up of genera which resemble 

 one another more than they resemble other genera, and their likeness is 

 again a sign of affinity. But to account for the greater difference 

 between the extreme individuals belonging to a family, evolution must 

 have had more time; that is, the common source of the members of a 

 family must have antedated the common source of the individuals of a 

 genus. Orders, classes, and phyla are similarly regarded as having 

 sprung from successively more remote ancestors, the time differences 

 being necessary to allow for the differences in the amount of evolution. 

 This statement is, however, only in a general way correct. Since 

 evolution has probably not proceeded at the same rate at all periods 

 or in all branches of the animal kingdom at any one time, the time rela- 



