effected with more or less ease, at periods more or less remote; on the
nature and number of the former immigrants; and on their action and
reaction, in their mutual struggles for life; the relation of organism
to organism 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 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,
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