190 Proceedings of the Royal Physical Society. 



The Arctic shell-beds of our own and other temperate regions 

 sufficiently prove that geographical conditions were not the 

 only factor concerned in bringing about the peculiar climate 

 of the Pleistocene period. Then, again, we must not forget 

 that at certain stages of the same period genial conditions of 

 climate were coincident with a much wider land-surface in 

 north-western Europe than now exists. The very fact that 

 interglacial deposits occur in every glaciated region is enough 

 of itself to show that the Arctic conditions of the Pleistocene 

 could not have resulted entirely from a mere elevation of 

 land in the northern parts of our hemisphere. 



The only explanation of the peculiar climatic vicissitudes 

 in question which seems to meet the facts, so far as these 

 have been ascertained, is the well-known theory advanced by 

 Dr Croll. After carefully considering all the objections 

 which have been urged against that theory, there is only one, 

 as it seems to me, that is deserving of serious attention. This 

 objection is not based on any facts connected with the 

 Pleistocene deposits themselves, but on evidence of quite 

 another kind. It is admitted that, were the Pleistocene 

 deposits alone considered, Croll's theory would fully account 

 for the phenomena. But, it is argued, we cannot take the 

 Pleistocene by itself, for if that theory be true, then climatic 

 conditions similar to those of the Pleistocene must have 

 supervened again and again during tlie past. Where, then, 

 we are asked, is there any evidence in Palaeozoic, Mesozoic, 

 or Cainozoic strata of former wide-spread glacial conditions ? 

 If continental ice-sheets, comparable to those of the Pleisto- 

 cene, ever existed in the earlier ages, surely w^e ought to find 

 more or less unmistakable traces of them. Now, at first sight, 

 this looks a very plausible objection, but it has always seemed 

 to me to be based upon an assumption that is not warranted 

 by our knowledge of geographical evolution. Dr Croll would 

 be the first to admit that high eccentricity of the earth's orbit 

 might have happened again and again without inducing 

 glacial conditions like those of the Pleistocene. The objection 

 takes no account of the fact that the excessive climate of the 

 glacial period was only possible because of special geographical 

 conditions — conditions that do not appear to have been fully 



