ISOLATION 289 



them are virtually unknown. In nature a closely re- 

 lated distinct species is not often quite side by side 

 with the old. It is simply next to it, geographically 

 or geologically speaking, and the degree of distinc- 

 tion almost always bears a relation to the importance 

 or the permanence of the barrier separating the sup- 

 posed 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 may 

 be familiar, or of which the actual history or the actual 

 ramifications may in some degree be traced." . . . 



"In regions broken by a few barriers, migration 

 and interbreeding being allowed, we find widely dis- 

 tributed 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 proc- 

 ess of adaptation by natural selection. In regions 

 broken by barriers which isolate groups of individuals 

 we find a great number of related species, though in 

 most cases the same region contains a smaller num- 

 ber of genera or families." 



"In these and in all similar cases," he adds in con- 

 clusion, "we may confidently affirm: the adaptive 

 characters a species may present are due to natural 

 selection or are developed in connection with the de- 

 mands of competition. The characters, non-adaptive, 



