ORGANIC EVOLUTION — JORDAN 323 



observation of many Avorkers. For it is a matter of common Imowl- 

 edge among field naluralists that the minor differences which sep- 

 arate species and subspecies are due to some form of isolation with 

 segregation. Selection produces adaptation, but the distinctive 

 charactei's of species (m-e, in general, nonada'ptive. By some barrier 

 or other the members of one minor group are prevented from inter- 

 breeding with those of another minor group or Avith the mass of the 

 species. As a result, local peculiarities are fixed. " Migration holds 

 species true, localization lets them slip," or rather leaves them in the 

 backwash of currents of evolution. Peculiarities thus set off by isola- 

 tion become intensified by in-and-in breeding, or segregation, and 

 the particular environment exercises some continuous type of selec- 

 tion until at last there emerges a new form, recognizable as distinct. 

 And while its range rarely coincides with that of the parent species, 

 or with any other closely cognate form, neither is it likely to be 

 located far aAvay. In the few cases where the range of geminate 

 species overlaps in any degree, the fact seems to find an explanation 

 in the surmounting (to some extent) of a barrier by one or other of 

 the twin forms. The obvious immediate element in the formation 

 and molding of species is therefore isolation, wdth (behind) the 

 factors of heredity, variation, selection, and others as yet more or 

 less hjq^othetical. 



Illustrations of geminate species of birds, mammals, fishes, rep- 

 tiles, snails, and insects are well known to all students of these 

 groups; examples may be found on every hand. I have myself 

 gathered the record of hundreds of pairs of zoological twins, an 

 enumeration for which the present paper has no space.^ 



It is clear enough that species change with space and Avith time. 

 With space, because separation takes place and new environment 

 brings new stress of selection while isolation of individuals involves 

 some difference in parentage. In like fashion, species change wnth 

 time, because new conditions arise, new enemies, new foods, new 

 separations, new selections. That notable differences obtain in time, 

 even in pure strains, and when there is no visible reason for change, 

 is clearly shown by the experience of stock breeders. In geologic 

 time one geminate species often follows another and in the same 

 locality, a fact lately shown by the writer in an extensive survey 

 of the Miocene fish fauna of California. - 



The application of Jordan's Law to plants has been denied. 

 But geminate species are just as prevalent in botany as in zoology,, 

 and the effects of isolation in species forming among plants are 



' See Dudley Memorial Volume, Stanford University Tress, 1913. 



^ Fossil Fisiies of the California Tertiary : Stanford University Itess^ t9-l. 



