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.[1]
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.[2]
Professor Whitney has lately put forth another theory, attributing the early warmth to the sun itself being hotter in geological times
- ↑ There probably were zones of climate in the latter half of the Tertiary, or at least in the Pliocene, but these are quite compatible with the absence of seasons, since, with a perpendicular axis, temperature, however it might differ on different parallels, would be constant in each.
- ↑ Since writing the above I have read Professor Woeikof's article in the "American Journal of Science" for March, 1886, entitled "An Examination of Dr. Croll's Hypotheses of Geological Climates." It is a careful testing of Dr. Croll's theories by applying them to present facts, as to summer and winter variations of climate. He shows that no such differences exist as Dr. Croll's theories demand. He sums up his conclusions (page 178) as follows: "The main points on which rests, so to say, the whole fabric in its explanation of glaciation and geological climates generally—the influence of winter in aphelion and in perihelion—during high eccentricity, and the calculation of temperatures in proportion to the sun-heat received, are unfortunately fallacious." The article will well repay the student of geological climate for its careful study.
