shrubs, to be found in it. . . . 2. The prairie-region succeeds; a grassy land, with many peculiar herbaceous American genera, including Mexican types, of which last the most conspicuous are a yucca and the cacti, which latter increase in number as the Rocky Mountains are approached, where they form a noticeable feature in the landscape. In the parks and lower valleys of the Rocky Mountains, deciduous trees are few and scattered, and the forest is an open one of conifers. . . . Higher on the mountains the coniferous forests are dense. . . . 3. Descending to the sink-region, . . . deciduous trees are very few and confined to the gullies of the mountains. . . . The hardy sage-brush (Artemisia) covers immense tracts of dry soil, and saline plants occupy the more humid districts. 4. The Sierra Nevada is clothed with the most gigantic coniferous forests to be found on the globe, among which a very few species of deciduous trees are scattered; but none of these are identical with trees of the Eastern forests."
Applying the geological charts to these four general floral regions, we find corresponding to each of them respectively: 1. The great Silurian and carboniferous beds, with their large varieties of deciduous trees, the Alleghanies on the east, with their coniferous plants, and the loess-beds on the west, with their peculiar prairie flora, and a few trees along the streams. 2. The deeper loess-beds with a peculiar flora, and the Rocky Mountains with their mixed geological characters, mainly volcanic, and with a mixed flora of Eastern and Western trees, the latter predominating. 3. The tertiary beds of the saline region, which axe different from those of the East, with their peculiar sage-brush and saline flora. 4. The Sierra Nevada region, with mixed geological characters of gneiss and lava, and a mixed Mexican and Northwest flora.
Thus, from more than one point of view, the North American flora is susceptible of being divided into three or more distinct floras, corresponding to the different geological formations which they inhabit.
| PERRIER ON THE THEORY OF DESCENT. |
By M. A. ESPINAS.
THE preface of twenty pages with which M. Edmond Perrier has introduced M. Leveque's French translation of Mr. Darwin's essay on "Earth-Worms" is a masterly work, the importance of which will escape no one. We know that this eminent naturalist, after having given a quite cool reception to the theory of descent, at last, in his "Colonies Animales," accepted it, under reserves, the tendency of which was to restrict the bearing of evolution at different points. First, he denied that that theory could explain the passage from the
