chap, iv.] EVOLUTION THE KEY TO DISTRIBUTION. 
63 
portions. This is the more likely to he 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 
