THE STORY OP THE EARTH 43 



by those who follow this theory to lend it a striking 

 confirmation. Such a claim is premature. In the long 

 story of the earth there must have been, on Dr. Croll's 

 principles, so many glacial periods that a very large 

 number must be discovered to establish his theory. On 

 the other hand, the geological record in the rocks is so 

 rough and so little explored as yet that the theory is by 

 no means discredited by the present scantiness of 

 evidence. However, most modern geologists look else- 

 where for the causes of periods of intense cold. Some 

 speak of atmospheric disturbance following volcanic 

 action on a large scale: others think a change in the 

 ocean currents might suffice. But the general feeling 

 converges upon two plausible agencies. One of these is 

 a purification of the atmosphere, and the other is the 

 rise of the land to a higher level. 



Both these causes were conspicuously at work during 

 or before the Permian period. It is calculated that our 

 coal-forests (and rocks that absorbed carbon) must have 

 taken from the Carboniferous atmosphere from 20,000 

 to 100,000 times the quantity of carbon dioxide that 

 there is in the actual atmosphere. When we find this 

 abnormal change succeeded by a great upheaval of land 

 we seem to have an adequate explanation of the ice-sheets 

 of which we find traces in the Permian strata. Some 

 geologists do not hesitate to say that these changes 

 represent a " revolution " in the face of the earth ; though 

 we must remember that they occupied probably four or 

 five million years. They certainly involved consequences 

 of the first importance for living things. The ice-clothed 

 areas imply much larger tracts with a lowered tempera- 

 ture, and there is hardly any agency in nature so 

 productive of biological changes as the lowering of 

 temperature. 



This revolution brings us to the close of the first or 



