238 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



drew attention to the eccentricity of the earth's orbit round 

 the sun, and the fact that during the summer season of the 

 southern hemisphere the earth is in its nearest position to the 

 sun (perihelion), while during the winter season of the same 

 hemisphere the earth is at its greatest distance from the sun 

 (aphelion). He then argued, since the eccentricity of the 

 orbit was variable, sometimes having the form of a long 

 ellipse, sometimes approximating to a circle, during the epochs 

 of greater eccentricity of the orbit, the hemisphere whose 

 winter falls in aphelion would undergo a protracted period of 

 winter cold. The climate might be thereby rendered so severe 

 that stupendous masses of ice would accumulate near the Pole 

 in aphelion, and as a further consequence the centre of gravity 

 of the earth might be shifted. According to Adhemar, the 

 conditions favourable for extensive glaciation recur in each 

 hemisphere at intervals of 10,500 years, and thus call forth 

 periodic Ice Ages. 



Although Sir John Herschel, Arago, and Humboldt were of 

 opinion that the eccentricity of the earth's orbit could have but 

 a slight influence upon the climate of our planet, Adhemar's 

 theory was accepted by Julien (1860) and Le Hon (1868) 

 with scarcely any modification. James Croll treated the 

 subject of cosmic causes of climatic variation in a memorable 

 work, Climate and Time (1875). He improved the theory 

 enunciated by Adhemar, inasmuch as he showed the depend- 

 ence of the prevailing winds and ocean-currents upon the 

 eccentricity of the earth's orbit, and explained how masses of 

 ice and snow accumulating at the Pole must, in virtue of their 

 radiation of cold, absorption of heat, and condensation of 

 moisture, tend strongly to reduce the temperature. Croll 

 supposed that the interglacial periods were characterised by 

 the almost complete withdrawal of the glacier ice, and by 

 extensive subaerial disturbance of the glacial deposits. In 

 Great Britain, Croll's views have been accepted by many 

 geologists, amongst others by Sir Archibald Geikie and his 

 brother, Professor Geikie. Professor Penck and Professor 

 Pilar are the best known of Croll's adherents on the 

 Continent. 



Sir Charles Lyell took objection to Croll's theory, mainly 

 because of the insufficient geological evidence of recurring 

 epochs of glaciation; nor can this objection be said to be even 

 yet overcome. Neumayr doubts, on the one hand, whether 



