If, then, we admit that when one portion of a species is separated from
the rest, there will necessarily be a slight difference in the average
characters of the two portions, it does not follow that this difference
has much if any effect upon the characteristics that are developed by a
long period of isolation. In the first place, the difference itself will
necessarily be very slight unless there is an exceptional amount of
variability in the species; and in the next place, if the average
characters of the species are the expression of its exact adaptation to
its whole environment, then, given a precisely similar environment, and
the isolated portion will inevitably be brought back to the same average
of characters. But, as a matter of fact, it is impossible that the
environment of the isolated portion can be exactly like that of the bulk
of the species. It cannot be so physically, since no two separated areas
can be absolutely alike in climate and soil; and even if these are the
same, the geographical features, size, contour, and relation to winds,
seas, and rivers, would certainly differ.
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