TJie Evohition of Climates. — Manson. 95 
Chapter II. 
Revieiv of the Prineipal Theories ^uJiich have been advanced 
to account for Glacial Epochs. 
Vie^us of Ejfiinent Geologists. It would not be instructive 
to attempt to review all of the theories which have been urged, 
as some writers have ascribed remote, inadequate or obscure 
causes, and others resorted to hypotheses beyond the range of 
either analysis or investigation; therefore, only the leading 
causes will be briefly mentioned.* 
I. — A decrease in the original heat of the globe. 
2. — Changes in the elevation of land, and consequent varia- 
tions in the distribution of land and water. 
3. — Changes in the obliquity of the axis of the earth. 
4. A period of greater moisture in the atmosphere. 
5. — ^^ariations in the amount of heat radiated by the sun. 
6. — A variation in the heat absorbing power of the sun's 
atmosphere. 
7. — Variations in the temperature of space. 
8. — A coincidence of an aphelion winter with a period of 
maximum eccentricity of the earth's orbit. 
9. — A combination of 8 and 2. 
10. — The views of Sir Robert Ball, LL. D., etc., as ex- 
pressed in his recent work, "The Cause of an Ice Age,"f Chap- 
ters 5 and 6.% 
*See also the Climate Controversy. S. V. Wood, Jr., Geo!. Mag., 
1876 and 1883. Climate and Time, Climate and Cosmology, James 
Croll; Island Life, Alfred Russell Wallace, F. R. S. etc. Philosoph- 
ical Magazine, May, 1864. British Association Reports, part 2, p. 11, 
1876. Proceedings Royal Soc, vol. XXVIII, p. ' 15. Quart. Jour. 
Geological Soc, Feb. 1878; Nature, July 4, 1878; Trans. Geological 
Soc. Glasgow, Feb. 22, 1877; The Ice Age in North America, Dr. Fred. 
G. Wright, Appendix by Prof. Warren Upham; Revolutions de la Mer. 
Deluges Periodiques, Alphonse Joseph Adhemar. Report Brit. Ass'n. 
1892, p. 708, J. W. Gray, F. R. S. and Percy F. Kendall, F. R. S.; 
The Cause of an Ice Age, Dr. Sir Robert Ball; The Great Ice Age, 
Ed. of 1894, Chap. XLIII. Prof. Dr. Geikie. 
tSee also The Date of the Last Glacial Epoch. Gen. Drayson, R. 
A. Science, Nov. 25, 1892. 
JThe tenth is not properly a theory to account for the Ice age, but 
rather a demonstration of a particular problem by Dr. Ball, F. R. S., 
etc., to the effect that 63 per cent, of the solar heat reaches either 
hemisphere during its summer exposure, and the remaining T,y per 
cent, during winter exposure. Nothing is added to the Physical 
Theory of Dr. Croll, nor does the demonstration in any way remove 
the objections which have been urged against Dr. Croll's views. 
