THE 



AMERICAN NATURALIST 



Vol. XLII February, 1908 No. 494 



THE LAW OF GEMINATE SPECIES 



PRESIDENT DAVID STARR JORDAN 

 Stanford University 



In "Evolution and Animal Life," by Jordan and 

 Kellogg (page 120), the following words are used: 



"Ci\en any species, in any region, the nearest related species is not 

 to be found in the same region nor in a remote region, but in a neigh- 

 boring district separated from the first by a barrier of some sort or at 

 least by a belt of country, the breadth of which gives the effect of 

 a barrier." 



Substituting the word "kind" for species in the above 

 sentence, thus including geographical subspecies, or nas- 

 cent species, as well as species clearly definable as such, 

 Dr. J. A. Allen accepts this proposition as representing 

 a general fact in the relations of the higher animals. To 

 this generalization Dr. Allen, in a late number of Science, 

 gives the name of "Jordan's Law." The present writer 

 makes no claim to the discovery of this law. The lan- 

 guage above quoted is his, but the idea is familiar to all 

 students of geographical distribution and goes back to 

 the master in that field, Moritz Wagner. 



This law rests on the fact that the minor differences 

 which separate species and subspecies among animals are 

 due to some form of segregation or isolation. By some 

 barrier or other the members of one group are prevented 

 from interbreeding with those of another minor group 

 or with the mass of the species. As a result, local pecul- 

 iarities arise. "Migration holds species true, localiza- 

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