138 



THE GEOLOGICAL HISTORY OF PLANTS. 



to that of the coal period is that of warm, temperate re- 

 gions in the southern hemisphere. It is not properly a 

 tropical flora, nor is it the flora of a cold region, but 

 rather indicative of a moist and equable climate. Still, 



Fig. 63. — Walchia imbricatula, S. N., Permian, Prince Edward Island. 



we must bear in mind that we may often be mistaken in 

 reasoning as to the temperature required by extinct 

 species of plants, differing from those now in existence. 

 Further, we must not assume that the climatal conditions 

 of the northern hemisphere were in the coal period at all 

 similar to those which now prevail. As Sir Charles Lyell 

 has shown, a less amount of land in the higher latitudes 

 would greatly modify climates, and there is every reason 

 to believe that in the coal period there was less land than 

 now. Further, it has been shown by Tyndall that a very 

 small additional amount of carbonic acid in the atmos- 

 phere would, by obstructing the radiation of heat from 

 the earth, produce almost the effect of a glass roof or con- 

 servatory, extending over the whole world. Again, there 

 is much in the structure of the leaves of the coal-plants, 

 as well as in the vast amount of carbon which they ac- 

 cumulated in the form of coal, and the characteristics of 

 the animal life of the period, to indicate, on independent 



