On Geological Climate. 67 



to those of intertropical regions, to exist in polar regions, is 

 more connected with the higher temperature which these re- 

 gions must have enjoyed, than with the quantity of light which 

 they received. For this temperature must have been much more 

 considerable, since it was sufficient for the largest land animals 

 which ever existed, the mastodons, elephants, and rhinoceroses. 

 This high temperature did not depend, as, indeed, is admitted 

 by M. Alphonse de Candolle himself, on a change of the incli- 

 nation of the ecliptic ; but has been due solely to the radiation 

 of the internal heat of the globe, which took place in geological 

 epochs. 



The high temperature, which not only the polar regions, but 

 also all other parts of the earth, enjoyed at that period, was, in 

 fact, produced by that which the interior of our planet trans- 

 mitted, since, in consequence of the cooling of its external crust, 

 the temperature of this surface is now nearly reduced to that im- 

 parted by the solar rays. Hence, on the one hand, the very 

 considerable heat of the interior of the globe, at small distances 

 from the surface, and on the other, the slow, but constant di- 

 minution of that temperature, enable us to conceive easily how 

 animals and vegetables of which analogous species now live only 

 in the warmest regions of our globe, formerly lived in the polar 

 regions. 



In fact, at the epoch when these plants of the ancient world 

 existed, the heat belonging to the globe itself being united to 

 the solar heat, rendered the temperature of different climates 

 much more elevated ; but this temperature seems to have dimi- 

 nished more rapidly at the poles than in temperate regions, and 

 more so in the latter than in the hotest regions of the earth. 

 Each of the terrestrial zones, then, has passed through the equa- 

 torial temperature after having been subjected to the effects of 

 another still more elevated, and has been reduced to a tempera- 

 ture hardly depending on any other cause than solar heat. 



Without doubt the temperature of the same point has varied 

 with extreme slowness, the general phenomena necessarily re- 

 gulating particular facts. But that does not prevent the places 

 where it has been highest from preserving for a longer time that 

 which belonged to them, which seems to have been the case, 



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