Page:Popular Science Monthly Volume 29.djvu/375

Rh The fact that the earth's axis has a different obliquity from that of the moon proves that a change occurred in the one or the other after their separation; and, since the moon remains so nearly in the normal position, it must have been the earth that was changed. The uniformity of biological conditions in all latitudes indicates that the present obliquity had not been attained in Archæan time, nor in Palæozoic, nor in Mesozoic, nor in the Eocene, nor in the Miocene, nor in the earlier Pliocene; then comes a blank during which the Glacial epoch came and went, and, when again the record begins to be legible, there are, for the first time in the world's history, indications of alternating seasons.

In view of all these facts, it seems most probable that, in that blank interval, the Glacial epoch, or, more largely, between the end of the Miocene and the beginning of the Champlain, that movement occurred which gave the earth seasons, unequal days and nights, and greatly enlarged its limits of inhabitability.

It requires no argument to show that an axis nearly perpendicular would account for the otherwise inexplicable evenness of geological climate. Although the Gulf Stream, or other currents, might bend the isotherms, the temperature at any point would, with such an axis, have remained constant. The conditions as to light and actinic force would have been the same everywhere, save the variation due to greater or less latitude. All this, however, is compatible with great cold; hence it remains to inquire why the polar climate was so warm. Many theories have been advanced to solve this problem. I have neither space nor time to discuss them now, and will only say that six or seven of the earlier ones are ably treated by Searles V. Wood, Jr., in the "Geological Magazine" for September and October, 1876; also by Dr. Croll, in his "Climate and Time." Dr. Croll's own theory I have discussed at large in "The Three Climates of Geology" ("Penn Monthly," June, July, and August, 1880), and have there pointed out what seem to me insuperable objections to it.

Professor Whitney has lately put forth another theory, attributing the early warmth to the sun itself being hotter in geological times