12 PLANT GEOGRAPHY 



day, except in the case of the slow-cooling lavas of 

 volcanic regions, or the neighbourhood of hot springs, 

 we need not consider the heated interior of the earth 

 as contributing to the temperature of the air; but can 

 attribute this entirely to the direct, or reflected, heat 

 of the sun. 



During the earlier portion of the history of the plant- 

 world of which we have any satisfactory records, how- 

 ever, this may not have been the case. There is among 

 the oldest well-known fossil plants, those of the rocks 

 below the Coal-Measures, so great a uniformity, no matter 

 from what part of the world they may have been col- 

 lected, as to suggest one world-wide, or well-nigh world- 

 wide, climate which may have been owing to the then 

 operative and unradiated primitive heat of the globe. 



This Lower Carboniferous Flora is followed in the 

 Southern Hemisphere by evidence of extreme and wide- 

 spread conditions of great cold, which seems to have 

 spread northwards, and to have produced a marked 

 change in the vegetation, which also extended north- 

 ward, so that representatives of the southern cold 

 conditions mingle with descendants of those of the 

 earlier hot conditions. 



In yet more modern times, geologically speaking, 

 there is again a restored uniformity of vegetation in 

 Jurassic times almost as marked as that of the Lower 

 Carboniferous. Again, in far more recent times times 

 little, if at all, antedating the advent of man upon the 

 earth there is proof of the existence in the Northern 

 Hemisphere of extreme cold extending far south of the 

 Arctic Circle, and lasting for a long, though apparently 

 intermittent, period; so that vegetation may well have 

 been driven to migrate southwards, to return, more or 

 less, as the cold abated. 



We cannot, therefore, assume that either the dis- 

 tribution of land and water, or that of climate, have in 

 past ages been the same as they are at present. 



