V 
NATURAL SELECTION 
119 
The Importance of Isolation. 
Isolation is no doubt an important aid to natural selection, 
as shown by the fact that islands so often present a number 
of peculiar species ; and the same thing is seen on the two 
sides of a great mountain range or on opposite coasts of a 
continent. The importance of isolation is twofold. In the 
first place, it leads to a body of individuals of each species being 
limited in their range and thus subjected to uniform condi¬ 
tions for long spaces of time. Both the direct action of the 
environment and the natural selection of such varieties only 
as are suited to the conditions, will, therefore, be able to 
produce their full effect. In the second place, the process of 
change will not be interfered with by intercrossing with other 
individuals which are becoming adapted to somewhat different 
conditions in an adjacent area. But this question of the 
swamping effects of intercrossing will be considered in another 
chapter. 
Mr. 1 )arwin was of opinion that, on the whole, the largeness 
of the area occupied by a species was of more importance than 
isolation, as a factor in the production of new species, and in 
this I quite agree with him. It must, too, be remembered, 
that isolation will often be produced in a continuous area 
whenever a species becomes modified in accordance with varied 
conditions or diverging habits. For example, a wide-ranging 
species may in the northern and colder part of its area become 
modified in one direction, and in the southern part in another 
direction ; and though for a long time an intermediate form 
may continue to exist in the intervening area, this will be 
likely soon to die out, both because its numbers will be small, 
and it will bo more or less pressed upon in varying seasons by 
the modified varieties, each better able to endure extremes of 
climate. So, when one portion of a terrestrial species takes to 
a more arboreal or to a more aquatic mode of life, the change 
of habit itself leads to the isolation of each portion. Again, 
as will be more fully explained in a future chapter, any 
difference of habits or of haunts usually leads to some modi¬ 
fication of colour or marking, as a means of concealment from 
enemies ; and there is reason to believe that this difference will 
be intensified by natural selection as a means of identification 
