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I believe, 1859 1860 1861 1866 1869
There is no evidence, 1872

in no 1859 1860 1861 1866 1869
of the existence of any 1872

If, for instance, 1859 1860 1861
If 1866 1869 1872

which stand in direct competition 1859 1860 1861
after having long competed 1866 1869 1872

migrate in 1859 1860 1861
in their old home, were to migrate in 1866 1869 1872

modified. 1859 1860
or at all modified. 1861 1866 1869 1872

and even
families,
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
each
the
individual in its complex struggle for life, so the
amount
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
other,
migrate in a body into a new and afterwards isolated country, they
would
will
be little liable to modification; for neither migration nor isolation in themselves
can effect
effect
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.
According to
On
these views, it is
obvious
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
are
have
descended from the same progenitor. In the case of those species, which have undergone during whole geological periods
but
but
little modification, there is not much difficulty in believing that they
may
may
have migrated from the same region; for during the vast geographical and climatal changes which
will
will
have supervened since ancient times, almost any amount of migration is possible. But in many other cases, in which we have reason to
believ
believe
that the species of a genus have been produced within comparatively recent times, there is great difficulty on this head. It