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. Biologically, the differences
are sure to be considerable. The isolated portion of a species will
almost always be in a much smaller area than that occupied by the
species as a whole, hence it is at once in a different position as
regards its own kind. The proportions of all the other species of
animals and plants are also sure to differ in the two areas, and some
species will almost always be absent in the smaller which are present in
the larger country. These differences will act and react on the
isolated portion of the species. The struggle for existence will differ
in its severity and in its incidence from that which affects the bulk of
the species. The absence of some one insect or other creature inimical
to the young animal or plant may cause a vast difference in its
conditions of existence, and may necessitate a modification of its
external or internal characters in quite a different direction from that
which happened to be present in the average of the individuals which
were first isolated.

On the whole, then, we conclude that, while isolation is an important
factor in effecting some modification of species, it is so, not on
account of any effect produced, or influence exerted by isolation _per
se_, but because it is always and necessarily accompanied by a change of
environment, both physical and biological. Natural selection will then
begin to act in adapting the isolated portion to its new conditions, and
will do this the more quickly and the more effectually because of the
isolation. We have, however, seen reason to believe that geographical or
local isolation is by no means essential to the diffe