Page:Popular Science Monthly Volume 25.djvu/84

76 and over all the great continents as rapidly as such other portions of the earth became by temperature, climate, and other conditions ready to receive and maintain it? Is there any one locality answering to these conditions, and yet of which it may be said, in a grander and truer sense than it was said of Rome, that all roads lead to and from it; not only highways diverging to every part of the world, but with vehicles upon them; seed-wagons running constantly in the direction of the most favorable distribution and to the remotest parts of the earth? Any locality so related to the topography of the whole earth as to render such extensive movements of plants and animals from it in all conceivable directions, and to all distances, not only easy and probable, but consistent with their present distribution? Is there anything in similarity of form, anatomy, structure, size, color, food, habits, habitat, longevity, modes of propagation, terms of gestation, and capacity for inter-breeding between certain flora and fauna of the Eastern Continents and the Western, which would suggest that many species and varieties so widely separated might have come originally from the same locality and ancestry? Are plants and animals always improved, developed, and rendered prolific more by being moved one way than another? Are the prevailing bottom currents of air and ocean in the direction of such favorable movements? Are cases of extermination and degeneration the result of a counter-movement, or a failure to make such favorable movements?

Many facts and considerations exist and may be presented pointing to a solution of these questions, and fairly answering some of them.

Let us consider, in the first place, the probable condition of the earth previous to the advent of any sort of life upon its surface. A large portion of those who have formed any intelligent opinions, in the light of modern thought and investigation, upon the subject of cosmogony, believe and hold very firmly that the earth was at one time an intensely hot globe—indeed, a molten mass—and that in the lapse of time it has cooled down by radiation to its present temperature. It is not at all necessary for the purposes of the present inquiry to examine the so-called nebular theory, nor even to ask when or how this globe became so heated, nor to what extent it has now become cooled, nor need we inquire whether the earth is now but a molten mass covered with a comparatively thin crust, or has cooled and hardened to its very center. It is important, however, to have it understood at the outset that the facts and considerations here presented are addressed to those, and those only, who have reached and adopted the conclusion that this globe, at some time in the process of its formation and development, passed through a fiery ordeal, that the primary rocks are of igneous formation, and that there are many other existing conditions and obvious facts which can not well be accounted for except upon the hypothesis that the whole earth was once a molten mass.