372 Dr. J. Croll on the Physical Theory 



schaffe, H. Habenicht, and other geologists have shown that 

 there are in North Germany three distinct boulder-clays — an 

 Upper, Middle, and Lower, with two series of interglacial 

 beds. In these interglacial beds have been found organic 

 remains which evidently indicate a mild and genial condition 

 of climate. The younger interglacial period (the one prior 

 to the last great extension of the ice) in all probability corre- 

 sponds to the last interglacial period of Scotland, England, 

 and Ireland. Interglacial beds belonging to the same period 

 have been found in Switzerland, Italy, Denmark, North Ame- 

 rica, and other places, all indicating a mild and equable con- 

 dition of climate. 



There is another class of facts, almost entirely overlooked, 

 which will doubtless yet prove even more conclusively the 

 warm character of interglacial periods. These facts will be 

 referred to w T hen we come to consider the question of warm 

 polar climates. 



It would be impossible within the limits of the present 

 paper to give even the briefest outline of the recent disco- 

 veries 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 per- 

 fectly 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 hippo- 

 potamus, the Elephas antiquns, 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 eccen- 

 tricity, 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 eccentricity, 

 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 penultimate ice-sheets was not so great as to warrant 

 the supposition of any considerable difference in the amount 

 of eccentricity at the two periods when these ice-sheets were 

 respectively developed. In short, if the last great ice-sheet 

 can be explained without the supposition of a high state of 

 eccentricity, then there does not appear to be any real necessity 



