MODIFICATION OF THEORY EXAMINED. 131 



Europe was under a still more severe state of glacia- 

 tion. The ice-sheet at this time advanced farther 

 south into England, and extended into North Germany 

 as far as Saxony. It is perfectly obvious that this 

 sheet must have destroyed all plant and animal life in 

 Scotland ; and before the country could have become 

 covered with vegetation and stocked with those inter- 

 glacial animals to which Mr. Wallace refers, the ice 

 must have disappeared and the climate become mild. 



Equally conclusive are the facts adduced by Mr. 

 Wallace in reference to the interglacial beds of 

 England. "In the east of England, Mr. Skertchly," he 

 says, "enumerates four distinct boulder-clays with 

 intervening deposits of gravels and sands. Mr. Searles 

 V. Wood, jun., classes the most recent (Hessle) boulder- 

 clay as 'Postglacial,' but he admits an intervening 

 warmer period, characterised by southern forms of 

 mollusca and insects, after which glacial conditions 

 again prevailed with northern types of mollusca. 

 Elsewhere Mr. Wood says : — ' Looking at the presence 

 of such fluviatile mollusca as Cyrena fluminalis and 

 Unio littoralis, and of such mammalia as the hippo- 

 potamus and other great pachyderms, and of such a 

 littoral Lusitanian fauna as that of the Selsea bed, 

 where it is mixed up with the remains of some of 

 those pachyderms, as well as of some other features, 

 it has seemed to me that the climate of the earlier 

 part of the Postglacial Period in England was possibly 

 even warmer than our present climate ; and that it 

 was succeeded by a refrigeration sufficiently severe to 

 cause ice to form all round our coasts, and glaciers to 

 accumulate in the valleys of the mountain districts.' " 

 That these fauna indicate a warm and equable con- 

 dition of climate is further evident from Mr. Wallace's 

 remarks : — " The fact," he says, " of the hippopotamus 



