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the rest of the organic world. Many of these would at once be exterminated,
while others, being relieved from competition, might flourish and become
modified into new species. Even more striking would be the effects when two
continents, or any two land areas which had been long separated, were
united by an upheaval of the strait which divided them. Numbers of animals
would now be brought into competition for the first time. New enemies and
new competitors would appear in every part of the country; and a struggle
would commence which, after many fluctuations, would certainly result in
the extinction of some species, the modification of others, and a
considerable alteration in the proportionate numbers and the geographical
distribution of almost all.

Any other changes which led to the intermingling of species whose ranges
were usually separate would produce corresponding results. Thus, increased
severity of winter or summer temperature, causing southward migrations and
the crowding together of the productions of distinct regions, must
inevitably produce a struggle for existence, which would lead to many
changes both in the characters and {117} the distribution of animals. Slow
elevations of the land would produce another set of changes, by affording
an extended area in which the more dominant species might increase their
numbers; and by a greater range and variety of alpine climates and mountain
stations, affording room for the development of new forms of life.

_Geographical Mutations as a Motive Power in Bringing about Organic
Changes._--Now, if we consider the various geographical changes which, as
we have seen, there is good reason to believe have ever been going on in
the world, we shall find that the motive power to initiate and urge on
organic changes has never been wanting. In the first place, every
continent, though permanent in a general sense, has been ever subject to
innumerable physical and geographical modifications. At one time the total
area has increased, and at another has diminished; great plateaus have
gradually risen up, and have been eaten out by denudation into mountain and
valley; volcanoes have burst forth, and, after accumulating vast masses of
eruptive matter, have sunk down beneath the ocean, to be covered up with
sedimentary rocks, and at a subsequent period again raised above the
surface; and the _loci_ of all these grand revolutions of the earth's
surface have changed their position age a