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 Climatal 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 very 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 winters; 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 
