i888] Far-reaching Conclusions 



among the small, non-migratory fishes minnows 

 and darters especially the species are different, 

 each river having its own kinds, with their nearest 

 relatives occurring in the next, not in the same, 

 stream. Similar conclusions in regard to fishes I 

 have since extended to animals and plants generally, 

 affirming that the nearest relative of any given 

 form is usually not found in exactly the same region 

 nor at a distance, but just on the other side of some 

 barrier to distribution. This general rule governing 

 the formation of distinct species by isolation or sep- 

 aration was afterward named "Jordan's Law" 1 by Dr. Jordan's 

 Joel A. Allen, late of the American Museum in New Law 

 York, a distinguished ornithologist who has instanced 

 many illustrations among birds and mammals. 



Another phase of this parallelism I designate as Geminate 

 the "Law of Geminate Species" that is, of species 

 "twin kinds" for whenever the range of a par- 

 ticular animal or plant form is split by a sharp 

 barrier, the individuals on either side may ultimately 

 be different enough to develop species technically 

 distinct though closely resembling each other. To 

 this fact thousands of illustrations bear witness. 

 Notable among them are the twin forms of fishes 

 and mollusks on opposite sides of the Isthmus of 

 Panama, which has separated them since the Miocene 



1 It is, of course, obvious that "Jordan's Law" is mine only for the sake of 

 convenience, and because I happened to be the first one to formulate it. It 

 exists primarily in the nature of things as a so-called "natural law" that 

 is, one of the many observed ways in which life proceeds. It concerns those 

 diversities in living forms which set off what we call "species." The term 

 has no absolute objective definition or criterion. A species of animal or plant 

 is in its origin merely one of the many kinds into which living organisms become 

 divided; once established, a species may then be defined as a particular series 

 of organisms giving rise by processes of reproduction to a continuous succession 

 of individuals not exactly alike, but so nearly alike that for the ordinary pur- 

 poses of science a single name serves for a whole group. 



n 329 3 



