CHAP. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 



63 



portions. This is the more hkely to be the case because the 

 most recently formed species, probably adapted to local condi- 

 tions and therefore most removed from the general type of the 

 group, will have the best chance of surviving, and these may 

 exist at several isolated points of the area once occupied by the 

 whole group. We may thus understand how the phenomena 

 of discontinuous areas has come about, and we may be sure 

 that when allied species or varieties of the same species 

 are found widely separated from each other, they were once 

 connected by intervening forms or by each extending till it 

 overlapped the other's area. 



Discontinuous Specific Areas, why rare, — But although dis- 

 continuous generic areas, or the separation from each other of 

 species whose ancestors must once have occupied conterminous 

 or overlapping areas, is of frequent occurrence, yet undoubted 

 cases of discontinuous specific areas are very rare, except, as 

 already stated, when one portion of a species inhabits an island. 

 A few examples among mammalia have been referred to in our 

 first chapter, but it may be said that these are examples of 

 the very common phenomenon of a species being only found in 

 the station for which its organisation adapts it ; so that forest 

 or marsh or mountain animals are of course only found where 

 there are forests, marshes, or mountains. This may be true, 

 and when the separate forests or mountains inhabited by the 

 same species are not far apart there is little that needs explana- 

 tion ; but in one of the cases referred to there was a gap of a 

 thousand miles between two of the areas occupied by the species, 

 and this being too far for the animal to traverse through an 

 uncongenial territory, we are forced to the conclusion that it 

 must at some former period and under different conditions have 

 occupied a considerable portion of the intervening area. 



Among birds such cases of specific discontinuity are very rare 

 and hardly ever quite satisfactory. This may be owing to birds 

 being more rapidly influenced by changed conditions, so that 

 when a species is divided the two portions almost always become 

 modified into varieties or distinct species ; while another reason 

 may be that their powers of flight cause them to occupy on the 

 average wider and less precisely defined areas than do the species 



