134 DISCUSSIONS IN CLIMATOLOGY. 



places, all indicating a mild and equable condition of 

 climate. 



There is another class of facts, almost entirely over- 

 looked, which will doubtless yet prove as conclusively 

 the warm character of interglacial periods. These 

 facts will be referred to when we come to consider 

 the question of warm polar climates. 



It would be impossible at present to give even the 

 briefest outline of the recent discoveries in regard to 

 interglacial periods. But though this were possible 

 it would be wholly unnecessary, as the facts which 

 have already been adduced by Mr. Wallace himself 

 are perfectly sufficient for our present purpose. 



If now it be true, as it undoubtedly is, that the 

 Hessle boulder-clay of England belongs to the same 

 age as the Upper Till of Scotland, and that the last 

 warm interglacial period — when the Cyrena fluminalis 

 and Unio littoralis, the hippopotamus, the Mephas 

 antiqwws, and other animals of a southern type lived 

 in England — occurred between two glacial periods so 

 severe as to envelop the greater part of North-western 

 Europe in a continuous sheet of ice, then this particular 

 interglacial period must have supervened during a 

 high state of eccentricity, and not, as Mr. Wallace 

 assumes, at a period subsequent to the Glacial Epoch 

 proper, when the eccentricity had greatly diminished. 

 This is obvious ; for if the last great ice-sheet could 

 have been produced without a high state of eccentri- 

 city, then there seems no reason why the one preceding 

 it should not also have been produced without high 

 eccentricity. If so, then all the previous ice-sheets 

 may in like manner have been so produced. For the 

 difference in magnitude between the last and penulti- 

 mate ice-sheets was not so great as to warrant the 

 supposition of any considerable difference in the 



