Let us suppose the zone comprised between the tropics to be occupied by land, and the portions without these limits to be covered with water. Under these conditions, the land, having a nearly vertical sun the whole time, would accumulate heat to a degree scarcely compatible with the existence of animal life. This is sufficiently proved by the oppressive tropical climates of the present time, influenced as they are by polar lands and contiguous seas. Under the same conditions, the sea would be heated by contact with the land, and the heat would be distributed by marine currents to the polar regions. But the water thus distributed would not part with its heat, because it has but little radiating power, and nowhere comes in contact with polar land. It follows, then, that both land and water would be subjected to a very high temperature. But, if we suppose the land confined to the polar regions, and the sea to the equatorial, the opposite results would follow. The equatorial sea would absorb but a small proportion of the solar heat which would be thrown upon it. The land would receive the sun's rays too obliquely to receive much elevation of temperature, as the present polar climates show. Hence, the temperature of the earth would differ but little from that of the planetary spaces, which is fifty-eight degrees below zero, a temperature too low to allow of any considerable development of organic life. These are the conclusions to which we are led by considering the different powers of land and water to absorb and radiate heat, and we shall find that the existing climates are in accordance with these conclusions. America has a lower temperature than Europe in the same latitudes. It has also a smaller proportion of land in the equatorial regions, and a greater proportion in the north polar regions. The eastern continent is colder in Asia than in Europe in the same latitudes. It has also less equatorial and more polar land. The southern is colder than the northern hemisphere at equal distances from the equator. There is also less land near the equator on the south side, and probably as much land around the south as the north pole. Hence, we see that there may have been such a relation of land and water as to account for all the variations of temperature which are known to have existed. We cannot say that such actually has been the case. We can tell, with some degree of accuracy, what portions of the present continents were land at the several geological periods; but three-fourths of the surface of the earth is covered with water, and of the condition of this portion during those periods we have no means even of conjecturing. We can only say, that, by the operation of known causes, the relative position of land and water may have been such as to produce the climates known to have existed at former periods of the history of the earth. INDEX. Page A.