fter age, so that each portion of
every continent has again and again been sunk under the ocean waves, formed
the bed of some inland sea, or risen high into plateaus and mountain
ranges. How great must have been the effects of such changes on every form
of organic life! And it is to such as these we may perhaps trace those
great changes of the animal world which have seemed to revolutionise it,
and have led us to class one geological period as the age of reptiles,
another as the age of fishes, and a third as the age of mammals.

But such changes as these must necessarily have led to repeated unions and
separations of the land masses of the globe, joining together continents
which were before divided, and breaking up others into great islands or
extensive archipelagoes. Such alterations of the means of transit would
probably affect the organic world even more profoundly than the changes of
area, of altitude, or {118} of climate, since they afforded the means, at
long intervals, of bringing the most diverse forms into competition, and of
spreading all the great animal and vegetable types widely over the globe.
But the isolation of considerable masses of land for long periods also
afforded the means of preservation to many of the lower types, which thus
had time to become modified into a variety of distinct forms, some of which
became so well adapted to special modes of life that they have continued to
exist to the present day, thus affording us examples of the life of early
ages which would probably long since have become extinct had they been
always subject to the competition of the more highly organised animals. As
examples of such excessively archaic forms, we may mention the mud-fishes
and the ganoids, confined to limited fresh-water areas; the frogs and
toads, which still maintain themselves vigorously in competition with
higher forms; and among mammals the Ornithorhynchus and Echidna of
Australia; the whole order of Marsupials--which, out of Australia, where
they are quite free from competition, only exist abundantly in South
America, which was certainly long isolated from the northern continents;
the Insectivora, which, though widely scattered, are generally nocturnal or
subterranean in their habits; and the Lemurs, which are most abundant in
Madagascar, where they have long been isolated, and almost removed from the
competition of higher forms.

_Climatal Revolutions as an Agent in Producing Organic Changes._--The