THE EVOLUTIONARY SIGNIFICANCE OF SPECIES." 



By O. F. Cook. 



The theory that species came into existence as results of separate 

 creative acts has given phice to the belief that the diversity of organic 

 nature has been attained by gradual transformation, but the uncon- 

 scious influence of the older view remains very strong. Even scien- 

 tific students of evolution still take it for granted that there should 

 be species, and hope to solve the problem of evolution by finding 

 special conditions or agencies which can make new species out of old 

 ones. 



Science is by no means exempt from the very human tendency to 

 place a fictitious value upon rare and exceptional phenomena, and to 

 overlook the significance of familiar facts. Between Linnseus and 

 Darwin a century of careful descriptive study of plants and animals 

 intervened before the everyday incident of variation was appreciated, 

 and it came to be perceived that species might arise through progres- 

 sive change, or evolution. Having once assumed, however, that evo- 

 lution could explain the origin of species, we have taken to seeking 

 for evolutionary causes in the out-of-the-way places of biology, for- 

 getting that the very existence and constitution of species affords 

 evidence of the most fundamental character regarding the nature of 

 the evolutionary process. 



Why are organisms grouped into species, instead of manifesting 

 unlimited and indefinite diversity? After reviewing the multiplicity 

 of organic types and the innumerable modifications possible for each, 

 it seems superfluous to doubt that the possibilities of diversity are 

 endless. And yet all higher organisms are associated in species, and 

 such associations have existed, a])parently, throughout the biological 

 history of the earth. Eacli investigator in turn has persevered in the 

 hope that in learning the origin of species he would unravel the secret 



a Consists principally of a revision and combination of two articles, " Evolu- 

 tion Not tbo Origin of Species" (Popular Science Monthly, Marcb, 1904) and 

 •"Natural Selection in Kinetic Evolution" (Science, April 1, 1904). 



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