232 GLACIAL EPOCHS. [CH. V 



United States. Water lilies, pond weeds, and an iris, 

 flourished at the same time in the same place. This 

 however does not indicate by any means the limits of 

 life towards the pole ; in Grinnel land distant only 

 8J degrees from the pole itself there was a flora composed 

 of poplars, elms, hazels and the swamp cypress again. 

 This warm period preceded the glacial period, which of 

 course put an end to the flora and fauna or else drove it 

 southwards. It is the opinion of many that the glacial 

 period was not an isolated phenomenon but the last in 

 a series of many glacial periods. If this were so 

 and if they recurred with tolerable frequency as is 

 again the opinion of many, all question of a polar 

 origin of life would be out of the range of possibility. 

 This view of the matter has been mainly based upon 

 two series of facts. In the first place it is argued that 

 these glacial periods have been due to the variations 

 in the obliquity of the earth's orbit, which alternates 

 from a more to a less pronounced ellipse. This would 

 bring about in conjunction with the rotation of the 

 earth on its own axis long cold winters and short hot 

 summers as well as the reversed condition. The former 

 would it is thought account for glacial periods. That 

 these have taken place in the recurrent fashion demanded 

 by this astronomical cause is attempted to be proved by 

 the existence of erratic boulders imbedded in the sedi- 

 mentary rocks belonging to past epochs. Mr Wallace 

 however holds that this cause is not of itself sufficient to 

 cause excessive glaciation; it is necessary also to have 

 continuous land surfaces and high mountains to retain the 



