400 THE EVOLUTIONARY SIGNIFICANCE OF SPECIES. 



EVOLUTION VERSUS TAXONOMY. 



The early evolutionists were all systematists deeply impressed by 

 the vast complexity of organic nature, a sentiment which ^^'e. may 

 still indulge, since two centuries of labor have but made a beginning 

 in the task of describing and assigning names to the millions of groups 

 of organisms. Nevertheless, it is to be regretted that biological evolu- 

 tion was viewed and expounded so exclusively from the standpoint of 

 the taxonomist. His interests are greatly at variance with those of 

 the evolutionist, and the confusion of the two distinct lines of investi- 

 gation has done much to perpetuate the erroneous identification of 

 the origin of species with the process of evolution. 



The evolution best appreciated by the taxonomist is one which pro- 

 duces species separable by definite and easily definable characters. 

 He finds such species on islands and in other circumsoi'ibed regions, 

 and infers that isolation is an important evolutionary factor, failing 

 to perceive that the " constancy " of insular species is merely a uni- 

 formity made possible by the limited area of distribution, and hence 

 usually absent in species of more extensive range. 



The systematist is prone to believe that there has been more evolu- 

 tion in a genus of ten readil}^ definable species than in another occupy- 

 ing the same geographical region, but consisting of only one species. 

 For the evolutionist, however, the segregation of the numerous species 

 means that the conditions are less uniformly favorable for the sub- 

 divided genus than for the other. Among fossil organisms, also, the 

 more generalized the types the wider was the distribution, the separa- 

 tion of local genera and species following with less favorable circum- 

 stances or greater competition. Segregation midtiplies 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 further 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 diver- 

 gence of characters is a deduction no more logical than that the differ- 

 ences of 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 taxonomic 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 liecome separate groups like other now segregated types, a supposi- 

 tion ([uite unsupported by evidence. Far more rational and more 

 secure would be the progress of systematic biology if recognition as 



