258 EVOLUTION AND GENETICS 



of 13,000 feet in southern Colorado and in the Appalachian range 

 as far south as North Carolina. The other is found at lower 

 altitudes in the western mountains and extends as far south as 

 Arizona. Such irregularity of distribution shows clearly how 

 transition from one side of a range to the other may be accom- 

 plished through the compensating influences of latitude and 

 altitude. 



Oceans are effective barriers to the migration of terrestrial 

 animals, as also are the climatic conditions of extensive desert 

 areas and to some extent, perhaps, other climatic extremes. To 

 volant species, however, such areas are not obstacles. Butterflies 

 have been observed three hundred miles from land and even the 

 normal seasonal migration of birds may carry them a much longer 

 distance over unbroken ocean. 



Aids to Dispersal. The passage of such areas is not impossible 

 to other organisms when aided by favorable circumstances. In 

 the geological past oceans have been bridged by narrow isthmuses 

 such as the Isthmus of Panama which now connects North and 

 South America. There was no such connection during most of the 

 Tertiary, consequently the relationships between the Nearctic and 

 Neotropical species are either older or more recent than that time. 

 On the other hand we know that Alaska was once joined to Asia 

 by an isthmus extending from the Alaskan peninsula to Kamchatka 

 which allowed free communication between the Nearctic and 

 Palearctic regions (Fig. 153). All that now remains above the 

 ocean is the chain of the Aleutian Islands, but the water between 

 them is nowhere very deep. The faunae are very similar on the 

 two continents because of the early association. Apparently the 

 remarkable Galapagos Islands were also once connected to the 

 Americas, but their isolation must have been remote, and even 

 the early occurrence of a connection is disputed. 



The land bridge between North America and Asia furnished a 

 convenient passage for many mammals during the Tertiary. 

 Extinct proboscideans and horses, as well as more obscure forms 

 which we have not considered, passed from continent to continent 

 over this route, and it is very probable that the ancestors of the 

 American Indian entered the continent from Asia in the same 

 way. 



Drifting debris and ice floes may also carry terrestrial animals 

 many miles over the ocean. Floating logs and masses of tropical 



