MiVTERIALS OF EVOLUTION 45 



and even when they j)i'oduce offspring, if the latter 

 are sterile, the two types are recognized as separate 

 species. It becomes necessary, therefore, to examine 

 further into the significance of these relationships. 



There are found in nature many interbreeding 

 mdividuals that are sharply defined from other 

 groups. It is convenient to have a name for all of the 

 individuals of such a group, even when, as in the 

 human species, the individuals living in the different 

 parts of the world may present striking differences 

 in structure, color, temperament, and social be- 

 havior. There are other types that are different from 

 all others within a circumscribed region, but in other 

 regions are rej)resented by individuals that show 

 slight but constant differences from the former. 

 Here it becomes a more difficult matter to decide 

 whether to call them all one species or whether to 

 make two species. In ^^ractice it is generally agreed 

 amongst taxonomists to give one specific name to 

 such groups, if intermediate forms between the 

 grouj)S are found, but, if intermediate types are 

 absent, the two separated groups are called differ- 

 ent species. It is obvious, therefore, that tlie dis- 

 tinction between such species is largely arbitrary 

 and artificial, especially since in the great majority 

 of cases no tests of cross-breeding are made to de- 

 termine whether the extreme types will cross and 

 leave fertile offspring, or whether they are infertile 

 or if fertile leave sterile offspring. 



