ISOLATION WITH SEGREGATION AS A FACTOR IN 

 ORGANIC EVOLUTION 



By David Starr Jordan 



The purpose of the present paper is to stress the fact known to 

 every field worker in zoology or botany, that the molding of species 

 is mainly due to the separation by barriers of one kind or another of 

 related forms. There is probably not a genus in which separate spe- 

 cies have arisen in nature, that does not show that these forms have 

 some sort of geographical basis. There is not a case on record in 

 which the origin of a distinct species in nature can be traced with 

 any probability to " mutation " or to Mendelian or other hybridism. 

 That striking mutations occur in nature, as well as in the garden is 

 well known. Selective breeding will carry these very far, but in 

 open competition such variants are either crowded out or else 

 swamped by interbreeding with the mass. No normal definition of 

 species can be drawn from variants of this kind. A species is a 

 recognizable kind of organism produced in the natural divergence 

 of life, and which has run the gauntlet of time, and which has en- 

 dured. The problem of the origin of species relates to forms which 

 have lasted. The study of impermanent variants, whether due to 

 mutation or to artificial selection and segregation is a matter of 

 great importance. Among other things it should throw light on the 

 origin of actual species, but the problems involved in the two cases 

 are quite distinct. 



To begin with, forms actually found in nature we must consider 

 as genuine species. Nothing can be more real than that which 

 really exists. Natural species, nevertheless, have as a rule indefinite 

 boundaries shading off into subspecies, geminate or representative 

 species, ontogenetic forms and the like, and may be variously altered 

 by artificial selection in conjunction with artificial segregation. All 

 their multifarious eccentricities command attention. As Darwin once 

 observed, such facts are fascinating to us " as speculatists, however 

 odious to us as systematists." They must be reckoned with, not 

 through speculation but by intimate understanding of actual realities. 

 But to extend our knowledge of a species we must ring the changes 

 on the variations to which it is susceptible. For the degree of vari- 

 ability is also a specific character. On such problems hundreds of 



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