120 THE PROBLEMS OF EVOLUTION 



variable species as the domestic dog, as has been 

 pointed out by Devaux.^ 



Organic isolation, however, demands a consid- 

 erable degree of established difference within the 

 species. It may further evolution but cannot be 

 expected to initiate it. Geographical isolation, on 

 the contrary, may split a species into groups with- 

 out the slightest activity on the part of the indi- 

 viduals involved. Rivers like the Missouri not 

 infrequently establish new channels by cutting 

 across narrow necks of land left by their meander- 

 ing curves. The resulting ox-bow lakes left in the 

 old curved channels contain remnants of the river 

 population, now destined to live in still water and 

 away from the flora and fauna of the parent stream. 

 The ancient seas which cut across North America 

 from north to south must have divided species into 

 eastern and western groups in many cases. And 

 the separation of islands in oceanic archipelagos, 

 or the peopling of newly-formed islands by immi- 

 gration from other land masses, must in every case 

 result in spatial isolation of related individuals. 



It is very probable that individuals isolated in 

 a restricted area by such processes will possess a 

 limited range of the variations characteristic of the 

 species to which they belong. Their descendants 

 can only display the characters or express the 

 potentialities of the isolated stock, for without 



6 Rev. gen. sci. pures et appL, Vol. XXXIX (10), pp. 299-306, 1928. 



