e epoch have been destroyed by denudation, and hence we have no
record of their existence.[193]

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.

[Illustration: FIG. 34.--Diagram illustrating the Geological
Distribution of Plants.]

The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
will illustrate our present knowledge of the development of the
vegetable kingdom in geological time. The shaded vertical bands exhibit
the proportions of the fossil forms actually discovered, while the
outline extensions are intended to show what we may fairly presume to
have been the approximate periods of origin, and progressive increase of
the number of species, of the chief divisions of the vegetable kingdom.
These seem to accord fairly well with their respective grades of
development, and thus offer no obstacle to the acceptance of the belief
in their progressive evolution.

_Geological Distri