590 CENOZOIC ERA— AGE OF MAMMALS. 



cause of the rigorous climate of high latitude regions in America and 

 Europe. Meanwhile, we may imagine that other regions, especially 

 tropical regions, were as warm as now. 



But admitting that increase in the area and height of polar lands 

 would increase the rigor of the climate, and decrease of area and height 

 of polar lands would moderate the climate of northern regions, and 

 that the amount of this effect it is difficult to estimate — yet the effect 

 was so enormous and so wide-spread that the cause, even when sup- 

 plemented by changes in the course of oceanic currents such as the 

 Gulf Stream, has seemed to most physicists and geologists to be in- 

 sufficient. They have cast about, therefore, for some other possible 

 cause, external to the earth itself — i. e., cosmical cause — to ex- 

 plain it. 



doll's Theory. — Nearly all theories of this kind are open to the 

 fatal objection that they attempt to account only for the cold, while 

 heat is just as necessary as cold. As, in a distilling apparatus, a boiler 

 is the necessary complement of a condenser, so for a Glacial epoch ex- 

 cessive evaporation by heat is the necessary condition of excessive con- 

 densation of snow by cold. What we want, therefore, is great difference 

 of temperature between summer and winter, and between equator and 

 poles. The only theory which meets this objection, and which is there- 

 fore entitled to serious attention, is that of Mr. Croll (embraced also by 

 Geikie and many other English geologists), which attributes it to the 

 combined influence of precession of the equinoxes and secular changes in 

 the eccentricity of the earth'' s orbit. By the former — viz., precession — 

 winter, which in the northern hemisphere occurs now when the earth is 

 nearest the sun (perihelion), is gradually in 10,500 years brought round 

 so as to occur when the earth is farthest off from the sun (aphelion). 

 The effect of this, it is claimed, would be to make longer and colder 

 winters, and shorter but hotter summers in the northern hemisphere, 

 such as occur now in antarctic regions. By the latter — viz., increasing 

 eccentricity — these effects, which are now small on account of the 

 nearly circular form of the earth's orbit, would become very great. At 

 the time of greatest eccentricity, the earth would be 14,000,000 miles 

 farther off from the sun in winter than in summer, the winters would 

 be twenty-two days longer and 20° colder, and the summers twenty- 

 two days shorter, but much hotter than now. 



Fig, 963 is a diagram representing the effect of precession. In A 

 we have the condition as it now exists — i. e., the north pole, x p, is 

 turned away from the sun, s (winter), at perihelion, and toward the 

 sun (summer) at aphelion. But the earth, rotating on its axis like 

 a spinning-top, does not maintain the same position of its axis — does 

 not sleep in its spinning — but wabbles on its center, the ends of the 

 axis describing a small circle (as shown by the dotted line) in 21,000 



