THE CLASSIFICATION OF ANIMALS 71 



similar species. Sometimes it includes but a single known 

 species. That is, a species may not have any other 

 species resembling it sufficiently to group with it, and so 

 it constitutes a genus by itself. If later naturalists should 

 find other species resembling it they would put these new 

 species into the genus with the solitary species. Each 

 genus of animals is given a Greek or Latin name, of a 

 single word. Thus the genus including the hairy and 

 downy woodpeckers is called Dryobates; and the genus 

 including the flickers is called Colaptes. But it is neces- 

 sary to distinguish the various species which compose the 

 genus Colaptes, and so each species is given a name which 

 is composed of two words, first the word which is the 

 name of the genus to which it belongs, and, second, a 

 word which may be called the species word. The species 

 word of the Yellow-shafted Flicker is auratus (the Latin 

 word for golden), so that its scientific name is Colaptes 

 auratus. The natural question. Why not have a single 

 word for the name of each species ? may be answered thus : 

 There are already known more than 500,000 distinct 

 species of living animals ; it is certain that there are no 

 less than several millions of species of living animals; 

 new species are being found, described and named con- 

 stantly; with all the possible ingenuity of the word- 

 makers it would be an extremely difficult task to find or 

 to build up enough words to give each of these species a 

 separate name. This is not attempted. The same 

 species word is often used for several different species of 

 animals, but never for more than one species belonging 

 to a given genus. And the names of the genera are 

 never duplicated. (There are, of course, much fewer 

 genera than species, and the difficulty of finding words 

 for them is not so serious.) Thus the genus word in the 

 two-word name of a species indicates at once to just what 

 particular genus in the whole animal kingdom the species 



