XIII 
THE GEOLOGICAL EVIDENCES OF EVOLUTION 
401 
that all these groups inhabited the lowlands, where there w r as 
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. 1 
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 Rocky Mountains 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 favourable to dicotyledons than to 
monocotyledons, and Ave may, therefore, well suppose that the 
former originated within such elevated areas, and Avere 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, Avhere they noAA r , proportion¬ 
ally, most abound, and where the conditions of altitude and a 
dry atmosphere Avere probably present at a very early period. 
1 “ On the Origin of the Flora of the European Alps,” Proc. of Roy. Geog. 
Society, vol. i. (1879), pp. 564-588. 
2 D 
