chap, xxiv.] SUMMARY AND CONCLUSION. 
505 
that denudation alone has lowered these mountains so much 
during the quarternary epoch, that they were previously of 
sufficient height to account for the glaciation of all of them, 
but this hardly needs refutation ; for it is clear that denudation 
could not at the same time have removed some thousands of 
feet of rock from many hundreds of square miles of lofty 
snow-collecting plateaus, and yet have left moraines, and 
blocks, and even glacial striae, undisturbed and uneffaced on 
the slopes and in the valleys of these same mountains. 
The theory of geological climates set forth in this volume, 
while founded on Mr. Croll’s researches, differs from all that 
have yet been made public, in clearly tracing out the compara- 
tive influence of geographical and astronomical revolutions, 
showing that, while the former have been the chief, if not the 
exclusive, causes of the long-continued mild climates of the 
Arctic regions, the concurrence of the latter has been essential 
to the production of glacial epochs in the temperate zones, as 
well as of those local glaciations in low latitudes, of which there 
is such an abundance of evidence. 
The next question discussed is that of geological time as 
bearing on the development of the organic world. The periods 
of time usually demanded by geologists have been very great, 
and it was often assumed that there was no occasion to limit 
them. But the theory of development demands far more ; for 
the earliest fossiliferous rocks prove the existence of many and 
varied forms of life which require unrecorded ages for their 
development — ages probably far longer than those which have 
elapsed from that period to the present day. The physicists, 
however, deny that any such indefinitely long periods are avail- 
able. The sun is ever losing heat far more rapidly than it can 
be renewed from any known or conceivable source. The earth 
is a cooling body, and must once have been too hot to support 
life ; while the friction of the tides is checking the earth’s rota- 
tion, and this cannot have gone on indefinitely without making 
our day much longer than it is. A limit is therefore placed to 
the age of the habitable earth, and it has been thought that 
the time so allowed is not sufficient for the long processes of 
geological change and organic development. It is therefore 
