competitors in their
own widely-extended homes will have the best chance of seizing on new
places, when they spread into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo further
modification and improvement; and thus they will become still further
victorious, and will produce groups of modified descendants. On this
principle of inheritance with modification, we can understand how it is
that sections of genera, whole genera, 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 modification; 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
conditions. 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 believing 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 is also obvious
that the individuals of the same species, though now inhabiting distant
and isolated regions, must