40 EVOLUTION 



temporary depression of the Devonian period the land 

 began to rise once more. Large but low-lying tracts of 

 land extended south from the original northern con- 

 tinents, and living things definitively settled on them. 

 This settlement belongs to the two following chapters, 

 and we need only recall here the extensive swamp forests 

 that gradually covered so much of the earth. In this 

 case the development of life led to physical changes in 

 the nature of the earth, which were to have a momentous 

 reaction on living organisms in the succeeding ages. 

 The vast forests began for the first time to reduce the 

 stifling quantity of carbon dioxide in the atmosphere. 

 There were as yet very few air-breathing animals, and the 

 predominant feature was the absorption of carbon by the 

 abundant vegetation. Our vast stores of coal give us a 

 sufficient idea of the quantity that was withdrawn from 

 the atmosphere. It was purified and prepared for a 

 great increase of land-animals, and at the same time the 

 rays of the sun were now enabled to penetrate and give 

 a stimulating impulse to the development of life. 



The inexpert reader will expect that the full penetra- 

 tion of the sun's rays to the surface of the earth would 

 cause a rise in temperature, but the truth is exactly the 

 reverse of this. The carbon-saturated atmosphere had 

 acted as a blanket for the earth's surface, and kept it 

 hitherto at a high temperature from pole to pole.* 

 Apart from some disputed traces of glacial action in the 

 Cambrian period, geologists are agreed that to the end 

 of the Carboniferous period the temperature was high 

 and uniform, and the air very moist. Then the clearing 



* Let us note in passing that the planetesimal theory rejects 

 the idea of the atmosphere being filled with carbon dioxide 

 from the start. The indubitable abundance of carbon in the 

 Carboniferous period is attributed to emission out of the 

 crust of the earth in volcanic eruptions. 



