REFERENCE TO THE ANTIQUITY OF MAN. 



395 



under inquiry. Some of the objections which have occurred to me 

 against comparing the present rate of denudation with that of past 

 times and the Glacial period I have given elsewhere *, so need not 

 repeat them here. E"or is it my intention to discuss Dr. Croll's theory 

 upon his own special grounds, which he has argued with infinite 

 ingenuity and presented in so attractive a form that, as I am well 

 aware, it has carried conviction to a large number of geologists. 

 That the causes he assigns have considerable value, there can, I think, 

 be little doubt, and they .may have an important bearing upon certain 

 geological problems, such as relate to those minor periods of low tem- 

 peratures with which we are all acquainted, as, for example, that 

 which seems in early Eocene times to have affected the marine fauna 

 of the Thanet Sands and the flora of Gelinden • but that they are 

 sufficient to account for the extreme cold of the Glacial period is 

 open to question. There are, as he himself observes, astronomers 

 and physicists who are of opinion that the climate of the globe 

 could never have been seriously affected by changes in the eccen- 

 tricity of its orbit. The point has been contested by Professor 

 Xewcomb, the Rev. E. Hill, and others, whose papers and 

 Dr. Croll's rejoinders should be consulted f. 



If the cold were due to this cause, how is it that, whilst the 

 lesser eccentricity of 240,000 — 80,000 years ago resulted in the 

 intense Glacial epoch of the Pleistocene period, the effects of the 

 greater and longer eccentricity of 980,000 — 720,000 years ago, 

 which should surely have resulted in a still more intense glacial 

 period^ has not left its traces in anterior geological series. But 

 there is no such evidence even in the later Tertiary period. 



It is, however, not only on the question of the efficiency of the 

 hypothesis to account for the facts, a point which I would leave to 

 astronomers and physicists to decide upon, but on the geological 

 question whether the necessar}' concordance exists between the 

 observed phenomena and the phenomena as they should be were 

 we to accept Dr. Croll's views, that we should judge of its applica- 

 bility. If we adopt the hypothesis it should follow : — 



1st. That at intervals during all geological time there would be 

 some periods during which a recurrence of similar glacial 

 conditions took place. 



2nd. That interglacial conditions would affect each pole alter- 

 nately, and that there should be in both hemispheres a suc- 

 cession of warm interglacial periods. 



3rd. That the commencement of the Glacial epoch should be 

 placed about 240,000 years back, and have a duration 

 of 160,000 years ; after this the amelioration of the climate 

 to its present condition, which involves a Postglacial period 

 of about 80,000 years. 



4th. That the age assigned to Palaeolithic Man, even if limited to 



* ' Geology,' vol. i. chapter vi. 



t Various papers in the 'American Journal of Science,' 'Philosophical 

 Magazine/ and ' Geological Magazine,' from 1876 to 1884. 



