The Evolution of Climates. — Ma?iso?i. gg 
J. Changes in the Obliquity of the Axis of the Earth. 
While slight changes in obliquity have occurred, and must 
continue to occur, the results are too slight and the distribu- 
tion of glacial phenomena is too general to warrant the accept- 
ance of such change as a prime cause.* 
Of this view lord Kelvin says, "There is in fact no evidence 
in geological climate throughout those parts of the world 
which geological investigation has reached, to give any indica- 
tion of the poles having been anywhere but where they are, 
at any period of geological time."t 
4. A Period of Greater Moisture in the Atmosphere. 
That earlier ages were characterized by a more abundant 
distribution of moisture than now prevails cannot well be con- 
troverted. But a glacial period is not only dependent upon 
evaporation but also upon temperatures suf^ciently low to 
cause condensation in the form of snow and to conserve this 
over long periods. Warm seas with an abundant evaporation 
and cold continental areas must have been the characteristic 
conditions at the inauguration of the Ice age, but there is 
every reason to conclude that at the culmination of that age 
the seas were intensely cold and consequently that evaporation 
was reduced to a minimum. 
The assignment of a period of greater moisture in the air 
as a cause, appears to be attaching too great importance to one 
of the conditions prevaiUng at the inauguration of the Ice age. 
The author will endeavor later on to make this more apparent. 
5. Variations 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. 
These are hypotheses resting on insufficient data. 
8. A Coincidence of a?i Aphelion JVifiter with a Period of 
Maximum Eccentricity of the Earth's Orbit. 
The eighth theory has been presented to the scientific 
*The Great Ice Age, Ed. 1894, p. 794. See also note on same page. 
fPopular Lectures and Addresses. Vol. II. p. 298. See also Brit. 
Assn. Report 1876. Part II. p. 11. Prof. Geo. Darwin, Trans. Royal 
Soc, Vol. 167, pp. 271-312. 
