254: THE GEOLOGICAL HISTORY OF PLANTS. 



tend to reduce the practical importance of Mr. Croll's 

 theory, on the other hand they tend to remove one of the 

 greatest objections against it namely, that founded on 

 the necessity of supposing that glacial periods recur with 

 astronomical regularity in geological time. They cannot 

 do so if dependent on other causes inherent in the earth 

 itself, and producing important movements of its crust. 



The third great cause of warmer climates in the past 

 is the larger proportion of carbon dioxide, or carbonic- 

 acid gas, in the atmosphere in early geological times, as 

 proved by the immense amount of carbon now sealed up in 

 limestone and coal, and which must at one time have been 

 in the air. It has been shown that a very small additional 

 quantity of this substance would so obstruct radiation of 

 heat from the earth as to act almost like a glass roof. If, 

 however, the quantity of carbonic acid, great at first, was 

 slowly and regularly removed, even if, as suggested by 

 Hunt, small additional supplies were gradually added 

 from space, this cause could have affected only the very 

 oldest floras. But it is known that some comets and 

 meteorites contain carbonaceous matter, and this allows 

 us to suppose that accessions of carbon may have been 

 communicated at irregular intervals. If so, there may 

 have been cycles of greater and less abundance of this 

 substance, and an atmosphere rich in carbon dioxide 

 might at one and the same time afford warmth and abund- 

 dance of food to plants. 



It thus appears that the causes of ancient vicissitudes 

 of climate are somewhat complex, and when two or more 

 of them happened to coincide very extreme changes might 

 result, having most important bearings on the distribu- 

 tion of plants. 



This may help us to deal with the peculiarities of the 

 great Glacial age, which may have been rendered excep- 

 tionally severe by the combination of several of the causes 

 of refrigeration. We must not suppose, however, that 



