DRIFT MARINE: FA UNA. 221 



rence in the inhabitants of the sea; and accordingly a more 

 northern character has been observed in the marine fauna of 

 the drift-deposits of Sweden, Scotland, and England. A northern 

 character has also been manifested in the drift-fauna of the 

 Mediterranean as far as Sicily. Forms of animal life belonging 

 to the extreme north descended towards the south, and were 

 afterwards driven back to the north. Hence most of them are 

 only found in the fossil state in the drift -deposits of the above- 

 mentioned countries, although some still exist at great oceanic 

 depths, or at places where cold springs burst through the sea- 

 bottom ; and thus the same phenomenon is reproduced by the sea 

 with which we have become acquainted in the case of land plants 

 and animals. Thus colonies of Norwegian Crustacea occur in 

 the gulf of Quarnero, in the Adriatic, and also colonies of 

 Arctic animals in the lakes of the Norwegian coast, which were 

 formerly united with the sea, and have been separated from it 

 by the upheaval of the land. 



On glancing over the numerous facts furnished by both 

 organic and inorganic nature. Prof. Heer is compelled to con- 

 clude that the warm Tertiary epoch was followed by a period in 

 which the climate was much colder than at present. Even 

 during the Tertiary epoch a gradual diminution of temperature 

 took place, as is proved by a comparison of the (Eningian flora 

 with that of the Lower Miocene ; and in Pliocene times the cli- 

 mate approximated to that of the present day (see p. 174). In 

 the subsequent drift-period the temperature fell at the time of 

 the greatest development of the glaciers several degrees below 

 the present mean. If the temperature were now to fall about 

 4 or 5 C. (or 7'2 or 9 Fahr.), the glaciers would again make 

 their way irresistibly down into the low country and spread over 

 the plains, and this would take place the more rapidly in propor- 

 tion to the moisture of the climate and the amount of aqueous 

 precipitation. Only a slight diminution of temperature, there- 

 fore, is requisite in order to explain the phenomena of the Gla- 

 cial epoch ; at the same time it is certainly remarkable that at 

 the very commencement of the Glacial-drift period the glaciers 

 spread over the plain of Switzerland and then again retreated, 

 and during thousands of years a quiet formation of peat took 

 place in the same country where the glaciers had formerly been, 



