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 and unequal settlement of the crust, has ridged up and folded the rocks, producing mountain chains. This process began in the earliest geological periods, and has been repeated at long intervals, the original lines of folding guiding those formed in each new thrust proceeding from the broad 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, while the weight of new sediments and the removal of matter from other areas by denudation, have been constantly producing local depression and elevation. 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 of 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, Wallace,[184] and many other writers.[185] Let us now place beside them some facts as to the present distribution of life, and of the agencies which influence it. [184] Wallace, "Geographical Distribution of Animals" and "Island Life." Second edition. [185] The writer has endeavoured to popularize these great results of geology in his work, the "Story of the Earth." Ninth Edition. London, 1887. They are often overlooked by specialists, and by compilers of geological manuals.