148 ISLAND LIFE part i 



mountains are of moderate height ; while during summer 

 the sun is three millions of miles nearer, and the heat 

 received from it is equivalent to a rise of 20° F. as compared 

 with the same season in the northern hemisphere. The 

 only important differences are : the open southern ocean, 

 the longer and colder winter, and the general low tempera- 

 ture caused by the south polar ice. But the great ac- 

 cumulation of south polar ice is itself due to the great 

 extent of high land within the Antarctic circle acted upon 

 by the long cold winter and furnished with moisture by 

 the surrounding wide ocean. These conditions of high 

 land and open ocean we know did not prevail to so great 

 an extent in the northern hemisphere during the glacial 

 epoch, as they do in the southern hemisphere at the 

 present time ; but the other acting cause — the long cold 

 winter — existed in a far higher degree, owing to the ex- 

 centricity being about three times as much as it is now. 

 It is, so far as we know or are justified in believing, the 

 only efficient cause of glaciation which was undoubtedly 

 much more powerful at that time ; and we are therefore 

 compelled to accept it as the most probable cause of the 

 much greater glaciation which then prevailed. 



Geogrophical Changes, how far a Cause of Glaciation, — 

 Messrs. Croll and Geikie have both objected to the views 

 of Sir Charles Lyell as to the preponderating influence of 

 the distribution of land and sea on climate ; and they 

 maintain that if the land were accumulated almost wholly in 

 the equatorial regions, the temperature of the earth's surface 

 as a whole would be lowered, not raised, as Sir Charles Lyell 

 maintained. The reason given is, that the land being 

 heated heats the air, which rises and thus gives off much 

 of the heat to space, while the same area covered with 

 water would retain more of the heat, and by means of cur- 

 rents carry it to other parts of the earth's surface. But 

 although the mean temperature of the whole earth might 

 be somewhat lowered by such a disposition of the land, 

 there can be little doubt that it would render all extremes 

 of temperature impossible, and that even during a period 

 of high excentricity there would be no glacial epochs, and 

 perhaps no such thing as ice anywhere produced. This 



