DIVIDING FACTORS. ISOLATION 435 



•species of fishes and other aquatic types different from those found 

 anywhere else in the world. As a young naturalist, I became interested 

 in the fauna of Lake Maxinkuckee in Indiana, a spring-fed lake that 

 is practically cut off from communication with other bodies of water. 

 While I was there, Evermann and Clark were making a very exhaustive 

 biological survey of this lake and discovered a surprising number of 

 species peculiar to this one body of water. Hundreds of similar situa- 

 tions doubtless exist all over the world. 



Populations of mountains. — Certain types of plants characteristic 

 of high mountains are commonly isolated from others of their kind 

 by valleys and by discontinuities in mountain chains. Patches of 

 lowland thus isolate patches of mountains; and the result is, as one 

 might expect, that each isolated high mountain chain has its own local 

 races or sub-species of plants. 



b) Isolation due to sheer distance apart. — If a species ranges all 

 over a wide extent of territory which is unbroken by barriers, there 

 may be isolation because of the great distances between extreme out- 

 lying sections of the range. Students of the evolution of great groups 

 of mammals, for example, consider that very large continental bodies 

 must have been the main theaters of evolution, because smaller land 

 bodies would not permit of diversification of types through isolation. 

 Thus the body of land known as "Holarctica," embodying Northern 

 Asia, Northern Europe, and Northern America, is believed to have 

 been the principal theater of mammalian evolution. In this great 

 area there was plenty of room for groups to become spatially isolated 

 even without positive barriers, and in this area it is believed that all 

 the original divergences occurred that gave origin to the princple sub- 

 divisions of mammals. Migration along many southern routes has 

 subsequently more completely isolated and fixed the various mam- 

 malian faunas of Southern Asia, Australasia, Africa, and South 

 America. 



The original divergences resulting from mere spatial isolation 

 would be the result of the inability of the most widely separated sec- 

 tions of a species to interbreed. If they failed to interbreed, mutations 

 occurring in one section would remain in that section and those oc- 

 curring in another section would likewise remain in that section. Thus 

 two independent evolutions would go on and local races or varieties 

 would result that in time would become separate species, especially if 

 they subsequently migrated along different paths to different isolated 

 regions. 



