554 r. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



REVIEW OF HYPOTHESES PROPOSED TO ACCOUNT FOR THE INITIATION OF 



GLACIAL ACTIVITIES 



In general.- — The broader problems that we have been considering nat- 

 urally include the several hypotheses that have been advanced from time 

 to time to account for the origin of giaciation. Most of these hypotheses 

 were proposed to account for the initiation of the great Pleistocene ice 

 age; but, as the evidence gradually accumulated and it became evident 

 that there had been numerous other refrigerations, the burden forced on 

 these hypotheses of explaining all glaciations became too heav}', and one 

 after another they appear to have failed of adequacy. I have ventured to 

 review several of the more prominent of these proposals and to point out 

 where and why they seem to fail. 



Shifting the position of the poles. — In order to account for the pres- 

 ence of tropical vegetation within the polar circles and of giaciation in 

 middle latitudes, it was perhaps natural and seemingly logical to postu- 

 late a wandering of the earth's axis of rotation within its body — a shift- 

 ing of the poles. This hypothesis was advanced nearly a century ago, 

 and for a time found many advocates among geologists, and more espe- 

 cially among biologists, to whom it furnished an easy explanation of 

 certain puzzling facts of distribution among plants and animals. They 

 failed to appreciate, or indeed to perceive, that this shifting of the earth's 

 axis is ^^really a problem of mathematics, as much as are the movements 

 of precession and orbital perturbations." Hence astronomers and mathe- 

 maticians have in general been opposed to hypotheses of polar migration, 

 and Lord Kelvin, and more especially George H. Darwin, have appar- 

 ently set the matter definitely at rest. Beyond a very slight possible 

 movement, involving at the outside not more than 10 degrees, that may 

 have taken place in the geographic position which the axis held at the 

 consolidation of the earth, they conclude that polar migration is mechan- 

 ically impossible. 



Barrell has recently published an elucidating review of the "Status of 

 hypotheses of polar wanderings," ^^ which he concludes as follows : 



"It would appear that the assumption of polar wandering as a cause of 

 climatic change and organic migrations is as gratuitous as an assumption of 

 a changing earth orbit in defiance of the laws of celestial mechanics. Unless 

 some wholly unsuspected forces are at work within the centrosphere, polar 

 wandering has no more basis in science than Symmes's imaginings of a hollow 

 earth. From all that is known at present the doctrine must be regarded as a 

 vagrant speculation, not a working hypothesis," 



36 .Toseph Ban-ell : Science, n. s., vol. 40, Sept. 4, 1914, pp. 333-340. 



