Dr. CrolVs Theory of Glacial and Warm Periods. 67 



regions. When the precession of the equinoxes brings round 

 the winter solstice to aphelion the condition of things in the 

 two hemispheres is reversed, and the north-east trades then 

 blow over upon the southern hemisphere, carrying the great 

 equatorial currents along with them. The warm water 

 being thus wholly withdrawn from the northern hemisphere 

 its temperature sinks enormously, and snow and ice begin 

 to accumulate in temperate regions. The amount of preci- 

 pitation in the form of snow in temperate regions is at the 

 same time enormously increased by the excess of the evapo- 

 ration in low latitudes resulting from the nearness of the 

 sun in perihelion during summer." {Climate and Cosmology, 

 15 and 16.) 



This is shortly the glacial theory as taught by Dr. Croll, 

 and it involves the conclusion that there has been throughout 

 geological time an alternate glaciation of each hemisphere, 

 ultimately caused by changes in the eccentricity of the 

 earth's orbit, and directly engendered by the greater amount 

 of warm water forced into each hemisphere by the alternately 

 greater potency of the Trade winds North and South of the 

 Equator. This being the theory, let us now examine it with 

 the care which its wide acceptance demands. 



I may at once say, that at least two of Dr. Croll's most 

 important postulates seem incontrovertible. 



His prime postulate is that the chief factor in deter- 

 mining the distribution of heat over the world is the 

 direction of ocean currents. This he emphasizes in many 

 ways. Inter alia, he says, " It will be seen that to 

 such an extent is the temperature of the equatorial regions 

 lowered, and that of high temperate and polar regions 

 raised, by means of ocean currents, that were they to cease, 

 and each latitude to depend solely on the heat received 

 directly from the sun, only a very small portion of the 

 globe would be habitable by the present race of beings. 

 This being the case, it becomes obvious to what an extent 



