THE PLEISTOCENE OR GLACIAL PERIOD. 431 



The Crollian hypothesis encounters further serious difficulties when 

 applied to the Permian glaciation of India, Australia, and South Africa, 

 because of their low latitudes. The effect of eccentricity should be 

 felt chiefly in the higher latitudes, and should be a vanishing quantity 

 in the tropical belt. It is not clear how glaciation in the vicinity of 

 the tropics could be explained on this basis, particularly in the Paleo- 

 zoic era, unless the postulates of the atmospheric theory be also intro- 

 duced to furnish favorable working conditions. 



Other astronomical hypotheses. — Attempts have been made to 

 found other theories on the eccentricity of the earth's orbit, and also 

 to found them on variations in the obliquity of the ecliptic ; but none of 

 these has gained much acceptance. They have not been worked out 

 with the care and detail which Croll gave to his hypothesis. They 

 encounter most of the difficulties of the Crollian hypothesis, but in 

 somewhat different forms. 



There have been speculations upon the possible passage of the 

 earth through cold regions of space, but there is no astronomical basis 

 for them. 



The recent determination of Langley and Abbot that the heat 

 emitted by the sun varies as much as 10% within a short period, is 

 very suggestive; but a short-period variation really has no direct appli- 

 cation to a problem which requires a variation-period of tens of thou- 

 sands, if not hundreds of thousands, of years. 1 



The hypothesis of a wandering pole. — It was early suggested that 

 the axis of the earth may have been shifting its geographic position 

 and that the Pleistocene glaciations were but polar glaciations Of the 

 existing type, distributed over northeastern North America and north- 

 western Europe by an excursion of the pole through 15° or 20° of lati- 

 tude. So long as the theory of a thin crust resting on a liquid nucleus, 

 and capable of sliding over it, perhaps under the differential influence of 

 the tidal pull, was accepted, the mechanical difficulties of this hypothe- 

 sis did not seem insuperable; but if an effective rigidity of the body 

 of the earth be accepted, as now seems almost necessary, the dynamic 

 obstacles become extremely formidable, for no agency capable of pro- 

 ducing such a change in the axis seems rationally assignable. When 

 a few years ago it was discovered that changes of latitude were actu- 



1 Astrophysical Jour., Vol. XIV, 1904, pp. 305-321. 



