ISOLATION AS A FACTOR IN 

 ORGANIC EVOLUTION 



BY 

 DAVID STARR JORDAN 



By isolation, segregation or separation as a 

 factor in evolution, we mean the failure of a por- 

 tion of one group or species to interbreed freely 

 with the rest of its kind. Such failure is due to 

 the presence of some barrier which prevents free 

 interminghng of individuals or to some condition 

 or group of conditions which sets certain indi- 

 viduals off from the mass of their kind. Through 

 separations of this sort race distinctions arise, 

 and in time by the same means the more profound 

 modifications which mark what we call species. 

 The occasion of divergence in most cases is f otmd 

 in geographical separation, the " raiimliche Son- 

 derung," on which such strong emphasis has been 

 justly laid by Moritz Wagner. It may again 

 be a separation of some other kind, as segrega- 

 tion, through the occupation of different tracts 

 within the same general area, or seasonal separa- 

 tion, as when flowers bloom or animals mate at 

 different times of the year. There are also forms 

 of physiological segregation. Self-fertilized 

 plants mate with their neighbors irregularly or 

 by chance, the pure species standing alongside of 



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