5 24 THE POPULAR SCIENCE MONTHLY. 



shrubs, to be found in it. . . . 2. The prairie-region succeeds ; a grassy 

 land, with many peculiar herbaceous American genera, including Mexi- 

 can types, of which last the most conspicuous are a yucca and the 

 cacti, which latter increase in number as the Rocky Mountains are ap- 

 proached, 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. De- 

 scending 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 oc- 

 cupy 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 decidu- 

 ous 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 pecul- 

 iar flora, and the Rocky Mountains with their mixed geological char- 

 acters, mainly volcanic, and with a mixed flora of Eastern and Western 

 trees, the latter predominating. 3. The tertiary beds of the saline 

 region, which ax - e 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, cor- 

 responding to the different geological formations which they inhabit. 



PERKIER 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 



