CHAPTER XII. GEOGRAPHICAL DISTRIBUTION.
2. SINGLE CENTRES OF SUPPOSED CREATION. (continued)
In discussing this subject we shall be enabled at the same time to consider
a point equally important for us, namely, whether the several species of a
genus which must on our theory all be descended from a common progenitor,
can have migrated, undergoing modification during their migration from some
one area. If, when most of the species inhabiting one region are different
from those of another region, though closely allied to them, it can be
shown that migration from the one region to the other has probably occurred
at some former period, our general view will be much strengthened; for the
explanation is obvious on the principle of descent with modification. A
volcanic island, for instance, upheaved and formed at the distance of a few
hundreds of miles from a continent, would probably receive from it in the
course of time a few colonists, and their descendants, though modified,
would still be related by inheritance to the inhabitants of that continent.
Cases of this nature are common, and are, as we shall hereafter see,
inexplicable on the theory of independent creation. This view of the
relation of the species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, who concludes that "every species
has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he
attributes this coincidence to descent with modification.
The question of single or multiple centres of creation differs from another
though allied question, namely, whether all the individuals of the same
species are descended from a single pair, or single hermaphrodite, or
whether, as some authors suppose, from many individuals simultaneously
created. With organic beings which never intercross, if such exist, each
species, must be descended from a succession of modified varieties, that
have supplanted each other, but have never blended with other individuals
or varieties of the same species, so that, at each successive stage of
modification, all the individuals of the same form will be descended from a
single parent. But in the great majority of cases, namely, with all
organisms which habitually unite for each birth, or which occasionally
intercross, the individuals of the same species inhabiting the same area
will be kept nearly uniform by intercrossing; so that many individuals will
go on simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate what
I mean: our English race-horses differ from the horses of every other
breed; but they do not owe their difference and superiority to descent from
any single pair, but to continued care in the selecting and training of
many individuals during each generation.
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