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 tim.e 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 



