THE LAW OF GEMINATE SPECIES.* 

 By DAVID STARR JORDAN, President of Stanford University. 



IN "Evolution and Animal Life," by Jordan and Kellogg (page 120), the 

 following words are used: 

 "Given 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 neighboring 

 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 nascent species, as well as species 

 clearly definable as such, Dr. J. A. Allen accepts this proposition as repre- 

 senting a general fact in the relations of the higher animals. To this gen- 

 eralization Dr. Allen, in Science, has given 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 geo- 

 graphical distribution and goes back to the master in that field, Moritz 

 Wagner. Dr. Wagner was one of the most clear-sighted and long-headed men 

 of the early evolutionists. In recognizing the potency of isolation in species- 

 forming he made the mistake, however, of not recognizing selection as the 

 basis of adaptation. 



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. Selection produces adaptation, but the distinctive characters 

 of species are in general non-adaptive. They find their origin in the different 

 currents of life which isolation makes possible. 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 

 peculiarities arise. "Migration holds species true, localization lets them 

 slip," or rather leaves them behind in the process of modification. The 

 peculiarities of the parents in an isolated group become intensified by in and 

 in breeding. They become modified in a continuous direction by the selec- 

 tion induced by the local environment. They are possibly changed in one 

 way or another by germinal reactions from impact of environment. At last 

 a new form is recognizable. And this new form is never coincident in its 

 range with the parent species, or with any other closely cognate form, neither 

 is it likely to be located in some remote part of the earth. Whenever the 

 range of two such forms overlaps in any degree, the fact seems to find an 



* Most of this paper was published in the American Naturalist, vol. 42, pp. 73-80, 

 February, 1908, under the same title. 



