Continental and Island Life [pp. 1-29]

CONTINENTAL AND ISLAND LIFE.
from the remote Huronian period to the Tertiary the American
land occupied the same position as at present, and that its
changes were merely changes of relative level as compared with
the sea, but which so influenced the ocean currents as to cause
great vicissitudes of climate.
Having thus endeavored, however roughly and imperfectly,
to define the nature, extent, and causes of the vicissitudes of
climate and other physical conditions on our continents, we shall
be in a position to consider their present state and the causes of
the distribution of their living inhabitants.
In speaking of continents and islands it may be as well to
remark that all the land existing, or which probably has at any
time existed, consists of islands great or small. It is all surrounded by the ocean. Two of the greater masses of land are, however,
sufficiently extensive to be regarded as continents, and from their
very extent and consequent permanence may be considered as
the more special homes of the living beings of the land. Two
other portions of land, Australia and the antarctic polar continent, may be regarded either as smaller continents or large
islands, but partake of insular rather than continental characters
in their animals and plants. All the other portions of land are
properly islands; but while these islands, and more especially
those in mid-ocean, cannot be regarded as the original homes of
many forms of life, we shall find that they have a special interest
as the shelters and refuges of numerous very ancient and now
decaying species.
The two great continents of America and Eurasia have been
the most permanent portions of the land throughout geological
time, some parts of them having always been above water, probably from the Laurentian age downward, tho at various times
they have been reduced to little more than groups of islands.
On them, and more especially in their more northern parts, in
which the long continuance of daylight in summer seems in
warm periods to have been peculiarly favorable to the introduction of new vegetable forms, and to the giving to them that
vigor necessary for active colonization, have originated the
greater number of the inhabitants of the qand.
Regarded as portions of the earth's crust, the continents are
areas in which the lateral thrust, caused by the secular contraction of the interior of the earth, has ridged up and folded the
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