Trans. N. Y. Ac. Sci. 90 April 14, 
the ecliptic, and that once in any position it can never change* ‘‘ by 
any force known to science.” To all of which those of us who are not 
astronomers can only answer—What you say may all be true, but, 
nevertheless, the axis has been tilted over to 234° by means of some 
force not known to science. And if we cannot tell how it occurred— 
an inability which extends toa great many things besides this—our 
business is to discover, if we can, when it occurred. It is not a ques- 
tion of possibilities, but of chronology. The uniformity of biological 
conditions in high and low latitudes indicates that the present obliquity 
had not been attained in Archzan, or in Palzozoic, or in Mesozoic 
times. A similar uniformity prevailed in the Tertiary till toward its 
close ; then comes a blank of unknown length, during which the Glacial 
epoch came and went, and then, when the record again begins to be 
legible, there are, for the first time in the world’s history, indications 
of zones of climate and of alternating seasons. This seems to fix the 
date of the increase of axial obliquity as corresponding to the Glacial 
epoch. 
It requires no argument to show that a perpendicular axis would 
account for the otherwise inexplicable evenness of ancient climate. 
Ocean currents might bend the isotherms, but at any place the tem- 
perature, whatever it was, would be the same all the year. The hours 
of light and darkness would be the same everywhere. 
This kind of uniformity is, however, compatible with great cold, 
and does not touch the question of a warm polar climate. Many 
theories have been proposed to account for the remarkable tempera- 
ture. Six or seven are ably treated by SEARLES V. WOOD, Jr., in the 
Geological Magazine for September and October, 1876, and by Dr. 
CROLL in Climate and Time. Dr. CROLL’s theory differs from those. 
To it there are what seem insuperable objections. I have discussed 
this theory somewhat fully in Penn Monthly for June, July, and August, 
1880, and shall not now repeat. : 
Professor WHITNEY has lately put forth another explanation, in which 
he attributes the preglacial warmth to the sun itself being hotter. All 
conclusions in regard to the sun’s former temperature must be hypo- 
thetical; but if it be a gaseous body, as suggested by Professor YOUNG, 
it has been growing hotter all the time it has been condensing.t To 
this, as to all other theories heretofore advanced, lies the objection that 
they ignore the uniformity which is such a remarkable feature of 
geological climate. 
* There is a small secular change of obliquity going on now, but it oscillates abouta mean 
position. 
+ A fact discovered by Mr. J. H. Lane. See American Four. Sctence, July, 1880; also, New- 
comb’s Astronomy, page 508. 
