52 LAND MAMMALS IN THE WESTERN HEMISPHERE 



reality of species was almost universally held, until the publica- 

 tion of Darwin's "Origin of Species" (1859) converted the 

 biological world to the evolutionary faith, which declares that 

 the only objective reality among living things is the individual 

 animal or plant. 



According to this modern conception, a species may be 

 defined as signifying a "grade or rank assigned by systematists 

 to an assemblage of organic forms which they judge to be more 

 closely interrelated by common descent than they are related to 

 forms judged to be outside the species " (P. Chalmers Mitchell). 

 The technical name of a species, which is either in Latin, or in 

 latinized form, is in two words, one of which designates the 

 genus (see below) and the other the particular species of that 

 genus, as, for example, Equus caballus, the species Horse, E. 

 przewalskii, the Asiatic Wild Horse, E. asinus, the species 

 Ass, etc. In order to identify a species, the genus to which 

 it belongs must be stated, hence the term, binomial system 

 of nomenclature, which Linnaeus introduced, becoming tri- 

 nomial when the name of a subspecies is added, a modern re- 

 finement on the older method. A very large species (i.e. 

 one which is represented by great numbers of individuals), 

 extending over a very large area, is often divisible into groups 

 of minor rank, as varieties, geographical races or subspecies. 

 Taking the species as the unit in the scheme of classification, 

 the varieties and subspecies may be considered as fractions. 



There is great difference of usage among writers on sys- 

 tematic zoology in the manner of applying the generally ac- 

 cepted concept of species, some making their groups very much 

 more comprehensive than others, according as they are 

 "lumpers" or "splitters," to employ the slang phrase. The 

 difficulty lies in the fact that there are no fixed and definite 

 criteria, by which a given series of individuals can be surely 

 distinguished as a variety, a species or a genus ; it is a matter 

 for the judgment and experience of the systematist himself. 

 The individuals of a species may differ quite widely among 



