Chap. XI. 
SINGLE CENTRES OF CREATION. 
351 
and even families are confined to the same areas, as is 
so commonly and notoriously the case. 
I believe, as was remarked in the last chapter, in no 
law of necessary development. As the variability of 
each species is an independent property, and will be 
taken advantage of by natural selection, only so far as 
it profits the individual in its complex struggle for 
life, so the degree of modification in different species 
will be no uniform quantity. If, for instance, a number 
of species, which stand in direct competition with each 
other, migrate in a body into a new and afterwards 
isolated country, they will be little liable to modifica¬ 
tion ; for neither migration nor isolation in themselves 
can do anything. . These principles come into play only 
by bringing organisms into new relations with each other, 
and in a lesser degree with the surrounding physical con¬ 
ditions. As we have seen in the last chapter that some 
forms have retained nearly the same character from an 
enormously remote geological period, so certain species 
have migrated over vast spaces, and have not become 
greatly modified. 
On these views, it is obvious, that the several species 
of the same genus, though inhabiting the most distant 
quarters of the world, must originally have proceeded 
from the same source, as they have descended from the 
same progenitor. In the case of those species, which 
have undergone during whole geological periods but 
little modification, there is not much difficulty in believ¬ 
ing that they may have migrated from the same region ; 
for during the vast geographical and climatal changes 
which will have supervened since ancient times, almost 
any amount of migration is possible. But in many other 
cases, in which we have reason to believe that the species 
of a genus have been produced within comparatively 
recent times, there is great difficulty on this head. It 
