66 On Geological Climate. » 



action of this fluid, or to the constant and permanent brilliancy 

 of the solar rays. But, in our regions, the time during which 

 the sun shines with all its brilliancy is followed by that during 

 which it is below the horizon, and consequently one part of the 

 earth is seen constantly illuminated during the twenty-four hours 

 which compose the day, owing to the alternations of days and 

 nights. In the polar regions the intensity of the light does not 

 experience any of these alternations during the six months of day; 

 also it is extremely probable that this constant intensity compen- 

 sates in some measure for what it wants in duration. Hence, 

 may it not be reasonably supposed, that if the polar regions had, 

 at the present day, the same temperature as that which they 

 had in geological epochs, they would still possess plants analo- 

 gous to those of intertropical countries, the quantity of light 

 which they receive being sufficient to make them grow with 

 vigour ? Besides, do we not see at the present time a great 

 number of trees and plants vegetating with vigour in climates, 

 where the light has little brilliancy, on account of the imperfect 

 solution of vesicular vapour ? Forests also are generally more 

 numerous and more extensive in the regions of the north, than 

 in countries to the south, where, however, the light is more in- 

 tense and more brilliant. The coffee-plant of Arabia, where 

 the purity of the sky is almost constant, transplanted to the An- 

 tilles, has acquired an increase of size and vigour which it did 

 not possess in its native country. On the other hand, there are 

 a great many plants which do not prosper, except when shaded 

 from a bright light, as under the shelter of the trees of forests ; 

 such, for instance, as our Equiseta, our Lycopodia, and the greater 

 number of our Ferns. Finally, are there not some plants which 

 are sufficiently acted on by a bright light, but one of short dura- 

 tion ? Can we forget that, on high mountains or in northern re- 

 gions, vegetation goes to a certain extent through the same circle 

 in three months, which in other places requires more than double 

 the time ? On the other hand, in the countries where the light 

 is the most brilliant, there is also a season during which it is 

 often very feeble and nearly absent ; it is also the period during 

 which vegetation is almost suspended. 



The essential condition, then, which enabled plants analogous 



