220 
ISLAND LIFE. 
[part t. 
Geographical changes would be still more important, and it 
is almost impossible to exaggerate the modifications of the 
organic world that might result from them. A subsidence of 
land separating a large island from a continent would affect the 
animals and plants in a variety of ways. It would at once 
modify the climate, and so produce a series of changes from this 
cause alone ; but more important would be its effect by isolating 
small groups of individuals of many species and thus altering 
their relations to the rest of the organic world. Many of these 
would at once be exterminated, while others, being relieved from 
competition, might flourish and become modified into new 
species. Even more striking would be the effects when two 
continents, or any two land areas which had been long separated, 
were united by an upheaval of the strait which divided them. 
Numbers of animals would now be brought into competition 
for the first time. New enemies and new competitors would 
appear in every part of the country ; and a struggle would 
commence which, after many fluctuations, would certainly result 
in the extinction of some species, the modification of others, 
and a considerable alteration in the proportionate numbers and 
the geographical distribution of almost all. 
Any other changes which led to the intermingling of species 
whose ranges were usually separate would produce corresponding 
results. Thus, increased severity of winter or summer tempera- 
ture, causing southward migrations and the crowding together 
of the productions of distinct regions, must inevitably produce 
a struggle for existence, which would lead to many changes both 
in the characters and the distribution of animals. Slow eleva- 
tions of the land would produce another set of changes, by 
affording an extended area in which the more dominant species 
might increase their numbers ; and, by a greater range and 
variety of alpine climates and mountain stations, affording 
room for the development of new forms of life. 
Geographical Mutations as a Motive Power in bringing about 
Organic Changes. — Now, if we consider the various geographical 
changes which, as we have seen, there is good reason to believe 
have ever been going on in the world, we shall find that the 
motive power to initiate and urge on organic changes has never 
