130 LAND MAMMALS IN THE WESTERN HEMISPHERE 



obstacle to the acceptance of the glacial theory was the sup- 

 posed fact that the Pleistocene glaciation was something quite 

 unique in the history of the earth, a violent aberration in 

 the development of climates. Now, however, we have every 

 reason to believe that at least three other and very ancient 

 periods had witnessed similar climatic changes and that "ice- 

 ages" were recurrent phenomena. This is not the place to 

 discuss or even to summarize the evidence which has convinced 

 nearly all geologists of the reality of Pleistocene glacial con- 

 ditions on a vast scale in Asia, Europe and, above all, in North 

 America. The reader who may wish to examine this evidence 

 will find an admirable presentation of it in Vol. Ill of the 

 ''Geology" of Professors Chamberlin and Salisbury. 



North America. — There has long been a difference of 

 opinion among students of the Pleistocene as to whether the 

 glaciation was single, or several times renewed. That there 

 were many advances and retreats of the ice, is not denied ; 

 the question is, whether there were truly interglacial stages, 

 when the ice altogether disappeared from the continent and 

 the climate was greatly ameliorated. The present tendency 

 among American and European geologists is decidedly in 

 favour of accepting several distinct glacial stages (Chamberlin 

 and Salisbury admit six of these) separated by interglacial 

 stages, and for this there are very strong reasons. While it 

 is out of the question to present the evidence for this conclusion 

 here, one or two significant facts may be noted. On the north 

 shore of Lake Ontario, near Toronto, are certain water-made 

 deposits, which rest upon one sheet of glacial drift and are 

 overlaid by another. The fossils of the aqueous sediments 

 are in two series, upper and lower, of which the older and 

 lower contains plants and insects indicative of a climate con- 

 siderably warmer than that of the same region to-day and 

 corresponding to the temperature of modern Virginia. In 

 the upper and newer beds the fossils show the return of cold 

 conditions, much like those of southern Labrador, and this 



