hysical conditions.
The degrees of dissimilarity will depend on the migration of the more
dominant forms of life from one region into another having been more
or less effectually prevented, at periods more or less remote--on the
nature and number of the former immigrants--and on the action of the
inhabitants on each other in leading to the preservation of different
modifications; the relation of organism to organism in the struggle for
life being, as I have already often remarked, the most important of
all relations. Thus the high importance of barriers comes into play by
checking migration; as does time for the slow process of modification
through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in their
own widely-extended homes, will have the best chance of seizing on new
places, when they spread out 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.

There is no evidence, as was remarked in the last chapter, of the
existence of any 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 each individual in its
complex struggle for life, so the amount of modification in different
species will be no uniform quantity. If a number of species, after
having long competed with each other in their old home, were to migrate
in a body into a new and afterwards isolated country, they would be
little liable to modification; for neither migration nor isolation in
themselves effect 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 or at all
modified.

According to these views, it is obvious that the several species of