70 On Geological Climate. * 



the geological epochs during which the ancient plants flourished 

 there ; hence the existence of these plants in these regions may 

 be conceived, without our having recourse to changes so consi- 

 derable as those which would have depended on the variation 

 or the important modification in the inclination of the ecliptic. 



The light which the polar regions enjoyed in ancient geologi- 

 cal epochs, during at least half the year, — light so much the more 

 intense, because the solar rays then traversed masses of air ex- 

 tremely heated and transparent, in consequence of the complete 

 solution of aqueous vapour, — was doubtless sufficient for the life 

 of these ancient vegetables. Its brilliancy and intensity during 

 this lapse of time, compensated then in some degree for its near- 

 ly total absence, and its entire want, at a certain period of the 

 year. 



This hypothesis is so much the more admissible, because 

 we are completely ignorant if these arborescent ferns, these 

 equisetaceae, these lycopodiaceae, of the coal formation, required a 

 large quantity of light, and because the larger quantity of the 

 species analogous to these last actually live in the most shaded 

 places. Besides, it is possible that it may have been the case 

 with these plants, as it is with many which still flourish, that 

 a too great brilliancy of light may have fatigued instead of 

 being of advantage to them. Such are, for example, nearly all 

 the species of the genus Mirabilis, and the Silene nocturna, 

 whose flowers expand only after the setting of the sun. Alpine 

 plants, those which live constantly in gloomy shady places, and 

 which, in consequence of this and other circumstances, cannot 

 be raised, and cannot be made to flourish in our gardens, are 

 also a proof of this. There are besides so many degrees in this 

 respect in the vegetable scale, that we may very well admit that 

 the trees and plants may have been able to live, as we have al- 

 ready remarked, in places alternately illuminated by a brilliant 

 light, or nearly totally deprived of its enlivening influence. 



In fact, all plants are far from requiring a large quantity of 

 light, and perhaps it is in this respect as in regard to the tem- 

 perature which is necessary for them. The difficulty, however, 

 does not exist relative to this last influence, since all the facts 

 show that these polar regions had, at the epoch when these ar- 

 borescent ferns, equisetaceae, and lycopodiaceae lived, a tempera- 



