Page:Darwinism by Alfred Wallace 1889.djvu/425

 that all these groups inhabited the lowlands, where there was not only excessive heat and moisture, but also a superabundance 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 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 we may, therefore, well suppose that the former originated within 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, proportionally, most abound, and where the conditions of altitude and a dry atmosphere were probably present at a very early period.