XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 401 



that all these groups inhabited the lowhinds, where there was 

 not only excessive heat and moisture, but also a super- 

 abundance of carbonic acid in the atmosphere — conditions 

 under which these groups had been developed, but which 

 were prejudicial to the dicotyledons. These latter are 

 supposed to have originated on the high table-lands and 

 mountain ranges, in a rarer and drier atmosphere in which 

 the quantity of carbonic acid gas was much less ; and any 

 deposits formed in lake beds at high altitudes and at such a 

 remote epoch have been destroyed by denudation, and hence 

 we have no record of their existence.^ 



During a few weeks spent recently in the Rocky Mountains, 

 I was struck by the great scarcity of monocotyledons and 

 ferns in comparison with dicotyledons — a scarcity due 

 apparently to the dryness and rarity of the atmosphere 

 favouring the higher groups. If we compare Coulter's Rocky 

 Mountain Botany with Gray's Botany of the Northern (East) 

 United States, we have two areas which differ chiefly in the 

 points of altitude and atmospheric moisture. Unfortunately, 

 in neither of these works are the species consecutively 

 numbered ; but by taking the pages occupied by the two 

 divisions of dicotyledons on the one hand, monocotyledons 

 and ferns on the other, we can obtain a good approximation. 

 In this way we find that in the flora of the North-Eastern 

 States the monocotyledons and ferns are to the dicotyledons in 

 the proportion of 45 to 100 ; in the Eocky IMountains they 

 are in the proportion of only 34 to 100 ; while if we take an 

 exclusively Alpine flora, as given by Mr. Ball, there are not 

 one-fifth as many monocotyledons as dicotyledons. These 

 facts show that even at the present day elevated plateaux 

 and mountains are more favoiu^able to dicotyledons than to 

 monocotyledons, and we may, therefore, Avell suppose that the 

 former originated Avithin such elevated areas, and were for 

 long ages confined to them. It is interesting to note that their 

 richest early remains have been found in the central regions 

 of the North American continent, where they now, proportion- 

 ally, most abound, and where the conditions of altitude and a 

 dry atmosphere were probably present at a very early period. 



♦ ^ "On the Origin of the Flora of the European AIjds, " Proc. of Roy. Geog, 

 Society, vol. i. (1879), pp. 564-588. 



2d 



