Page:Popular Science Monthly Volume 16.djvu/477

 if these latitudes at this time were disposed as they are now—that is, if the earth's axis has not been displaced—all the earth received more heat, and the line of the tropics must have risen toward the north. The difference of the Miocene period may be valued at 25° or 30° of latitude—that is, we must at present descend 40° or 45° to find the temperature which then existed in Greenland.

The study of the more ancient floras brings new proof of this phenomenon of the extension of heat into the higher latitudes, and conducts us finally to that equality of climate of which we have spoken. "We are forced to conclude, however," remarks Saporta, "that when we reach the time of the coal and the most remote period in the history of organic beings, if there has been no change in the relations of the heat that falls upon our globe, there have doubtless been other changes profound enough to impress upon it a very different aspect from that which it has since presented, and to create conditions of existence about which we can form no idea." We are, in fact, ignorant of the conditions in which living beings first made their appearance and were developed. There have been many hypotheses about it, but the facts on which they rest are yet neither sufficiently numerous nor convincing. For the settlement of this question we must await the future.

As to the cause of climatic equality over all the earth in the Primary and Secondary epochs, we are equally in the dark. All the explanations that have been given have been successively rejected. The displacement of the axis of the earth, the inclination of that axis on the orbit of our planet, the precession of the equinoxes, etc., are some of the hypotheses put forth on this subject. We can not here enumerate them all; but there is one on which Saporta insists, not because it explains everything, but because it agrees more or less with the celebrated cosmogony of Laplace, and accords with the phenomena of the primitive world as revealed by science. This hypothesis was put forth some years ago by M. Blaudet. We know that, according to the theory of Laplace, the entire solar system was originally an immense nebula which has since condensed little by little, and successively given off rings of cosmical matter which have become planets. The central star is hence more and more reduced, has become more dense, more luminous and more ardent, until it has attained the dimensions and properties of our actual sun. In other words, if we could trace backward the course of the ages, we should find the sun progressively augmenting in volume, but its heat and light would diminish in intensity in the same proportion. We do not know what sun lighted the earth when life first appeared upon it, but, from the theory of Laplace, we may suppose that it was much larger than ours.

Such conditions, however, would explain many phenomena. This great sun, occupying a good part of the horizon, would give a twilight so luminous and so prolonged as perhaps to annul the night. Sending