THE DARBONIFEROUS AGE. 123 



when imperfectly known, have puzzled botanists in 

 regard to their position in one or other series. In 

 the present world, the flora most akin to that of 

 the Coal period is that of moist and warm islands 

 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. 

 In accordance with this is. the fact that the equable 

 but not warm climate of the southern hemisphere 

 at present (which is owing principally to its small 

 extent of land) enables sub -tropical plants to extend 

 into high latitudes. In the Coal period this uni- 

 formity was evidently still more marked, since we 

 find similar plants extending from regions within the 

 Arctic circle to others near to the tropics. Still 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 argued, 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. It has been shown by Tyndall that 

 a very small additional amount of carbonic acid in 

 the atmosphere would, by obstructing the radiation 

 of heat from the earth, produce almost the effect 

 of a glass loof or conservatory, extending over tlm 



