OTHER THEORIES OF SPECIES-FORMING 279 



will be that, if they differ in" any definite way from the main body of 

 the species, a new elementary species will at once gain a foothold and 

 will evolve independently of the parent-species. If a certain area of 

 land is cut off from a continent so as to form a continental island, the 

 members of each species that have become isolated will evolve independ- 

 ently of the main body of the species and will have their own peculiar 

 lines of variation preserved from back-crossing with the parent-species. 



Professor David Starr Jordan,* the leading proponent of the theory 

 of geographic isolation in America says: 



"It is now nearly forty years since Moritz Wagner (1868) first 

 made it clear that geographic isolation {raumlicke Sonderimg) was a 

 factor or condition in the formation of every species, race, or tribe of 

 animal or plant we know on the face of the earth. This conclusion 

 is accepted as almost self-evident by every competent student of 

 species or of the geographical distribution of species. But to those 

 who approach the subject of evolution from some other side the 

 principles set forth by Wagner seem less clear. They have never been 

 confuted, scarcely ever attacked, so far as the present writer remem- 

 bers, but in the literature of evolution of the present day they have 

 been almost universally ignored. Nowadays much of our discussion 

 turns on the question of whether or not minute favorable variations 

 would enable their possessors little by little to gain on the parent stock, 

 so that a new race 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 the parent. In theory, 

 either of these conditions might exist. In fact, both of them are 

 virtually unknown. In nature a closely related distinct species is not 

 often quite side by side with the old. It is simply next to it, geo- 

 graphically or geologically speaking, and the degree of distinction 

 almost always bears a relation to the importance or the permanence 

 of the barrier separating the supposed new stock from the parent 

 stock. 



"A flood of light may be thrown on the theoretical problem of the 

 origin of species by the study of the probable, actual origin of species 

 with which we are familiar or of which the actual history or the actual 

 ramifications may in some degree be traced. 



"In regions broken by few barriers, migration and interbreeding 

 t)eing allowed, we find widely distributed species, homogeneous in their 

 character, the members showing individual fluctuation and climatic 



' Science, N.S., Vol. XXII (1905). 



