EVOLUTION NOT THE ORIGIN OF SPECIES. 447 



genus than for the other. Among fossil organisms, also, the more gen- 

 eralized the types the wider was the distribution, the separation of local 

 genera and species following with less favorable circumstances or 

 greater competition. Segregation multiplies species by separating 

 groups of organic individuals, just as the ocean might form many 

 islands from a partially submerged continent. Species are biological 

 islands, but we do not go farther in biology than in geography by the 

 discovery that islands must be isolated. Isolation permits evolutionary 

 progress to be made on separate lines until the differences become of 

 diagnostic utility to the systematist, but that isolation is responsible 

 for the changes which bring about the divergence of characters is a 

 deduction no more logical than that the differences between islands are 

 due to the waters which separate them. 



Too narrow zeal in the descriptive task has led many systematists 

 to act on the assumption that the same amount of difference should 

 everywhere receive the same systematic recognition, a method some- 

 times defended on the ground that all variations of form or structure 

 indicate incipient species budding out from the parent stock, and sure 

 to become separate groups like other now segregated types, a supposition 

 quite unsupported by evidence. Far more rational and more secure 

 would be the progress of systematic biology if recognition as species 

 were limited to groups of individuals separate in nature, regard being 

 given to the completeness of segregation rather than to the amount of 

 difference. 



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

 locality offer tangible differences from any previously known, the work- 

 ing systematist must describe and name them as representing new 

 species. To crowd them into an old species by 'emending the descrip- 

 tion' or by calling them a 'variety' is to guess at an integration 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 tech- 

 nical 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 subdivisions of such groups 

 as 'species' shows how easily conventional taxonomic methods may ob- 

 scure 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 facilities for 

 interbreeding. A widespread species of sedentary animals or plants 

 will become locally diversified; more frequent intercommunication per- 



