Continental and Island Life [pp. 1-29]

THE PRINCE TON RE VIE W.
rocks, producing mountain-chains. This process began in the
earliest geological periods, and has been repeated at long in tervals, the original lines of folding guiding those formed in
each new thrust proceeding from the flat oceanic areas. Along
the ridges thus produced, and in the narrower spaces between
them, the greater part of the sediment carried by water was
laid down, thus producing plateaus in connection with the mountain-chains. The tendency of the ocean to be thrown toward
the poles by the retardation of the earth's rotation, alternating
with great collapses of the crust at the equator, proceeding from
the same cause along with the secular cooling, have produced
alternate submergence and emergence of these plateaus. This has
been further complicated by the constant tendency of the arctic
and antarctic currents, aided by ice, to drift solid materials, set free
by the vast denuding action of frost, from the polar to the temperate regions, and by the further tendency of animal life to
heap up calcareous accumulations under the warm waters of the
tropical regions. All these changes, as already stated, have conspired to modify the directions of the great oceanic currents, and
to produce vicissitudes of climate under which animals and plants
have been subjected in geological time to those migrations, extinctions, and renovations of which their fossil remains and present distribution afford evidence.
Still it is true that throughout the whole of these great
mutations, since the beginning of geological history, there seems
never to have been any time when the ocean so regained its
dominion as to produce a total extinction of land life, still less
was there any time when the necessary conditions for all the
various forms of marine life failed to be found; nor was there
any climatal change so extreme as to banish any of the leading
forms of life from the earth. To geologists it is not necessary
to say that the conclusions sketched above are those that have
been reached as the results of long and laborious investigation,
and which have been illustrated and established by Lyell, Dana,
and many other writers. They are on the whole fairly stated by
Wallace in his work on "Island Life."' Let us now place beside
1 The writer has endeavored to popularize these great results of geology in his
work the "Story of the Earth." They are often overlooked by specialists and by
compilers of geological manuals.
I2