chap, iv.] EVOLUTION THE KEY TO DISTRIBUTION. 
61 
that species vary in these more important as well as in the 
more superficial characters. If, then, in any part of the area 
occupied by a species some change of habits becomes useful 
to it, all such structural variations as facilitate the change will 
be accumulated by natural selection, and when they have be- 
come fixed in the proportions most beneficial to the animal, we 
shall have the first species of a new genus. 
A creature which has been thus modified in important 
characters will be a new type, specially adapted to fill its 
place in the economy of nature. It will almost certainly 
have arisen from an extensive or dominant group, because 
only such are sufficiently rich in individuals to afford an ample 
supply of the necessary variations, and it will inherit the 
vigour of constitution and adaptability to a wide range of 
conditions which gave success to its ancestors. It will there- 
fore have every chance in its favour in the struggle for existence ; 
it may spread widely and displace many of its nearest allies, 
and in doing so will itself become modified superficially and 
become the parent of a number of subordinate species. It 
will now have become a dominant genus, occupying an entire 
continent, or perhaps even two or more continents, spreading in 
every direction till it comes in contact with competing forms 
better adapted to the different environments. Such a genus 
may continue to exist during long geological epochs ; but the 
time will generally come when either physical changes, or 
competing forms, or new enemies are too much for it, and it 
begins to lose its supremacy. First one then another of its 
component species will dwindle away and become extinct, till 
at last only a few species remain. Sometimes these soon follow 
the others and the whole genus dies out, as thousands of genera 
have died out during the long course of the earth’s life-history ; 
but it will also sometimes happen that a few species will 
continue to maintain themselves in areas where they are removed 
from the influences that exterminated their fellows. 
Cause of the Extinction of Species . — There is good reason to 
believe that the most effective agent in the extinction of species 
is the pressure of other species, whether as enemies or merely 
as competitors. If therefore any portion of the earth is cut off 
