chap., till.] THE CAUSES OF GLACIAL EPOCHS. 
143 
important differences are : the open southern ocean, the longer 
and colder winter, and the general low temperature caused by 
the south polar ice. But the great accumulation 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 excentricity 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. 
Geographical 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 
currents 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 would result from there being no 
land near the poles to retain snow, while the constant inter- 
change of water by means of currents between the polar and 
tropical regions would most likely prevent ice from ever forming 
in the sea. On the other hand, were all the land accumulated in 
the polar and temperate regions there can be little doubt that a 
