THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 401 



species were limited to groups of individuals separate in nature, 

 regard being given to the completeness of segregation rather than to 

 the amount of diti'erence. 



It is to be admitted, of course, that when specimens from a new 

 locality offer tangible differences from any previously known, the 

 working systematist nuist describe and name them as reprsenting 

 new species. To crowd them into an old species by " emending the 

 description," or by calling them a " variety," is to guess at an integra- 

 tion in advance of knowledge; while to refuse to unite "species," 

 which have been shown to belong to a continuous series in nature, is 

 to prefer technical fiction to biological reality. A coherent group of 

 interbreeding individuals is the unit of evolutionary biology to which 

 the term species finds its most proper application. The tendency of 

 some systematists to refer also to intergrading unsegregated subdivi- 

 sions of such groups as " species," shows how easily conventional 

 taxonomic methods may obscure evolutionary distinctions. 



CRITERIA OF SPECIFIC DISTINCTNESS. 



Species differ, of course, in the variability of their characters, but, 

 other things being equal, the uniformity of the individuals of a 

 species might be expressed by a ratio between the range and the facil- 

 ities for interbreeding. A widespread species of sedentary animals 

 or plants will become locally diversified; more frequent intercom- 

 jnunication permits more uniform progress. A single species may 

 have as great a variety of characters as a dozen related groups which 

 have been segregated. Two species may be quite distinct and yet 

 differ much less than the connected extremes of another. That a 

 species differs in different parts of its range does not necessarily mean 

 that a subdivision will take place; it means merely that characters 

 are originating more rapidly than they spread over the whole species. 

 The integrity of a species is not destroyed by " inconstancy " of char- 

 acters, but because geographical or other barriers make a gap in the 

 series. 



The failure of the extremes of a widely distributed species to breed 

 when brought together does not prove the attainment of specific dis- 

 tinctness nor the approach of it, since internal diversity does not 

 weaken the species, but is an evolutionary advantage, and both ex- 

 tremes may continue to cross freely with the connecting forms which 

 constitute the bulk of the species. Neither does the power to form 

 fertile hybrids prove that two species occupying distinct ranges are 

 one. Faith in such criteria is simply a remnant of the preevolution- 

 ary theory of the separate creation of species. The only w^ay to ascer- 

 tain that two groups of organisms are separate species is to find the 

 SM 1904 26 



