25 



by Nordenskiold. 

 lows : — 



The oldest vegetable forms found fossil in the Arctic regions 

 are Sigillaria, Calamites, Lepidodendra, and other plants cha- 

 racteristic of the Coal period, and showing a warm climate ; 

 and the corals and shells bear the same witness. In the 

 Jurassic strata Cycads and Conifers are found, the representative 

 species of which now grow in the neighbourhood of the tropics. 

 During the Cretaceous period, a great change occurred. In the 

 lower Cretaceous periods, the characteristic forms are Ferns, 

 Cycads, and Conifers ; but before the conclusion of the period 

 these had given place to deciduous trees and other Dicotyledons, 

 among which are a Ficus and two species of Magnolia. In the 

 Miocene beds, besides plants belonging to a warm climate, we 

 find trees characteristic of a temperate climate like that of 

 Central Europe at present, such as limes, oaks, beeches, planes, 

 and large-leaved birches. 



These facts appear to show a progressive fall of temperature, 

 indicated first by the transition from Ferns and Cycads to 

 deciduous trees, and afterwards by the disappearance of these 

 latter, giving place to the present Arctic flora. The fall of 

 temperature, as Heer remarks, was probably accompanied by 

 a clearing away of cloud at the transition from Ferns to 

 Dicotyledons. 



I cannot however agree with Heer and Nordenskiold that there 

 ever was a time, since life began, when all latitudes had the 

 same climate ; but it is probably true that when the solar heat was 

 the greatest the difference of climate between different latitudes 

 would be the least, because the amount of evaporation increases 

 very rapidly with increasing temperature, and the higher 

 latitudes would be warmed by the heat liberated in the con- 

 densation of vapour which had been evaporated in the lower 

 latitudes. Moreover, the heat of the equatorial regions would 

 be mitigated by a veil of cloud screening the earth. But if all 

 latitudes had the same temperature, there would be no cause to 

 produce atmospheric currents between different latitudes, and 



* See the Getlogkal Magazint, November, 1875. 



