INTERNAL HEAT OF THE EARTH. 267 



The first attempt at explaining these striking changes of cli- 

 mate was by the hypothesis of a decrease in the heat belonging 

 to the earth. It was supposed that at its origin the earth had 

 been a fused mass, which gradually became refrigerated in the 

 cold medium that surrounded it. It is very probable that during 

 the early Palaeozoic and Carboniferous epochs the heat appertain- 

 ing to the earth exerted a great influence upon climate. From 

 the Carboniferous period down to the present day a slow and 

 uniform decrease of temperature ought to have taken place : 

 there are, however, no facts in confirmation of this hypothesis ; 

 for the flora and fauna of the Jurassic period and of the Creta- 

 ceous period bear no traces of any such diminution of tempera- 

 ture. It is only in the Miocene epoch that the modification 

 became strongly marked ; and at that geological period the 

 temperate zone, and still more the Arctic zone, required so great 

 an amount of heat that it was impossible to borrow it from the 

 internal heat of the earth, seeing that the Miocene epoch, in 

 point of time, is much nearer to the present time than to the 

 Jurassic period, and it is still more distant from the Carbonifer- 

 ous epoch. Between the Miocene epoch and the present time 

 a Glacial period occurred, indicating that during this interme- 

 diate epoch the temperature was considerably lower than that of 

 the present day, at least in the northern hemisphere. If this 

 phenomenon had been restricted to certain countries, it mij>ht 

 have been in some degree explained by a different distribution 

 of land and sea, and by changes of level. Very probably during 

 the drift-epoch the land in the north of Russia was lower by 

 some hundreds of feet, and, in consequence of this, the Arctic 



haye disappeared ; and their disappearance cannot be attributed to man, as 

 the game which then inhabited the virgin forests of Switzerland is still 

 abundant. The temperature of Switzerland was probably not higher than 

 at the present day, as is shown by the presence of the mountain-pine (Pinus 

 montana), the lowest region of which now in Switzerland is near the town of 

 Zurich. Since the period of the lake-dwellings, the climate seems in general 

 to have remained the same until the present time. Colder years alternate 

 with warmer years ; and this alternation is observable for a series of years, so 

 as to produce an advance or retreat of the glaciers in the Alps ; but these are 

 merely inconstant variations, which do not enable us to infer profound and 

 secular modifications of the climate of Europe. 



