288 THE THEORIES OF EVOLUTION 



individuals, and what Romanes designates as "apog- 

 amy" there is another form, designated as "homog- 

 amy," which fosters more directly the development of 

 a new type. In homogamy, all of the individuals 

 making up the isolated groups possess one certain 

 character which distinguishes them from the others; 

 they may have adopted a new mode of life or consume 

 different food or they may present physiological or 

 psychological divergences. Natural selection and ar- 

 tificial selection are but two of the forms of isolation 

 among heterogeneous individuals; they isolate the fit 

 from the unfit, those that present a desirable charac- 

 ter from those that lack it; thus these two general 

 factors are only manifestations of a more general 

 factor, i. e., isolation. 



One of the most ardent upholders of the species- 

 forming potency of geographical isolation at the pres- 

 ent day is David Starr Jordan, the foremost Amer- 

 ican student of the distribution and classification of 

 fishes. "Nowadays," he writes, "much of our discus- 

 sion turns on the question of whether or not minute 

 favourable variations would enable their possessors 

 little by little to gain on the parent stock, so that a 

 new species would be established side by side with the 

 old, or on whether a wide fluctuation or mutation 

 would give rise to a new species which would hold its 

 own in competition with its parent. In theory, either 

 of these conditions might exist. In fact, both of 



