DIVIDING FACTORS. ISOLATION 433 



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 

 being allowed, we find widely distributed species, homogeneous in their 

 character, the members showing individual fluctuation and climatic 

 effects, but remaining uniform in most regards, all representatives 

 slowly changing together in the process of adaptation by natural 

 selection. In regions broken by barriers which isolate groups of indi- 

 viduals we find a great number of related species, though in most cases 

 the same region contains a smaller number of genera or families. In 

 other words, the new species will be formed conditioned on isolation, 

 though these same barriers may shut out altogether forms of life which 

 would invade the open district. 



"Given any species in any region, the nearest related species is not 

 likely 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. 



"Doubtless wide fluctuations or mutations in every species are 

 more common than we suppose. With free access to the mass of 

 the species, these are lost through interbreeding. Isolate them as in 

 a garden or an enclosure or on an island, and these may be con- 

 tinued and intensified to form new species or races. Any horticul- 

 turist will illustrate this. 



"In all these and in similar cases we may confidently affirm: The 

 adaptive characters a species may present are due to natural selection 



