CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 



153 



hemisphere it is impossible to say. It may be that existing 

 geographical and physical conditions are there such potent agents 

 in producing a state of glaciation that no change in the phases 

 of precession would materially affect it. Still, as the climate of 

 the whole southern hemisphere is dominated by the great 

 mass of ice within the Antarctic circle, it seems probable that 

 if the winter were shorter and the summer longer the quantity 

 of ice would slightly diminish ; and this would again react on 

 the northern climate as already fully explained. 



The essential 'principle of Climated change restated. — The pre- 

 ceding discussion has been somewhat lengthy, owing to the 

 varied nature of the facts and arguments adduced, and the 

 extreme complexity of the subject. But if, as I venture to 

 hope, the principle here laid down is a sound one, it will be 

 of the greatest assistance in clearing away some of the many 

 difficulties that beset the whole question of geological climates. 

 This principle is, briefly, that the great features of climate are 

 determined by a combination of causes, of which geographical 

 conditions and the degree of excentricity of the earth's orbit 

 are by far the most important ; that when these combine to 

 produce a severe glacial epoch, the changing phases of pre- 

 cession every 10,500 years have ver}^ little, if any, effect on 

 the character of the climate, as mild or glacial, though it may 

 modify the seasons ; but when the excentricity becomes moderate 

 and the resulting climate less severe, then the changing phases 

 of precession bring about a considerable alteration, and even 

 a partial reversal of the climate. 



The reason of this may perhaps be made clearer by consider- 

 ing the stability of either very cold or very mild conditions, 

 and the comparative instability of an intermediate state of 

 climate. When a country is largely covered with ice, we may 

 look upon it as possessing the accumulated or stored-up cold of 

 a long series of preceding v/inters; and however much heat 

 is poured upon it, its temperature cannot be raised above the 

 freezing point till that store of cold is got rid of — that is, till 

 the ice is all melted. But the ice itself, when extensive, tends 

 to its own preservation, even under the influence of heat ; for 

 the chilled atmosphere becomes filled with fog, and this keeps 



