42 EVOLUTION 



of the atmosphere allowed a freer radiation of heat into 

 space, and so led to a lowering of the temperature. 



But further great changes were in progress at the 

 end of the Carboniferous and during the Permian period 

 which contributed to the fall of the earth's temperature. 

 A great uplifting of the crust was taking place in various 

 parts of the earth. Africa and South America now 

 definitely appear on the geological map, and two further 

 (and now lost) continents rise which connect Africa with 

 Brazil on the one hand, and with India and Australia on 

 the other. A chain of mountains (the Hereynian) surges 

 upward from Brittany to Bohemia, and in America a 

 more formidable chain (the Appalachian) rises and lifts 

 the land adjoining it. The land-surface of the earth was 

 now very considerable, the changes in the distribution of 

 land and water had a profound effect on their living 

 populations, and the climatic changes were not less 

 stimulating. For the first time we find at least plausible 

 traces of climatic zones and seasons, and In the higher 

 lands we get the first confident traces of glacial action. 



As far as present evidence goes In geology we have 

 disputed traces of glaciation in the Cambrian period, 

 certain traces in the Permian, and overwhelming proof 

 that in comparatively recent times a vast ice-sheet 

 covered the greater part of Europe and the northern 

 part of America. What the cause was of this repeated 

 appearance of colossal ice-caps outside of what are now 

 the polar regions (though even they then had semi- 

 tropical vegetation) geologists are by no means agreed. 

 The most popular theory, so to say, is the familiar one 

 of Dr. Croll, that the earth's axis slowly and periodically 

 changes its position, so that the pole and polar cap is 

 slowly and periodically displaced. The recent discovery 

 of glaciation in the Permian period and the claim of 

 discoveries of glacial action in other periods is thought 



