ause which alone, as
far as we positively know, produces organisms quite like, or, as we see in
the case of varieties, nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct
influence of different physical conditions. The degree of dissimilarity
will depend on the migration of the more dominant forms of life from one
region into another having been 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, {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 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 nearl