Thomas Jefferson Howell

“ THE causes which have determined the present distribution of the flora of the world have occupied the minds of some of the ablest students of natural history, but no satisfactory solution of the problem has yet appeared. If we accept the theory of Raumer, that plants are limited in their northern extension by heat alone, we shall find many anomalies difficult to reconcile, as no isothermal lines limit species. ”

“ The mountain-region of the Rocky Mountain province is composed of granite, but has enough trachyte and other volcanic rocks to modify its flora to some extent. It also has some beds of Silurian on the eastern border, and here, strongly corroborating our views, we find some of the Eastern flora mixed with the Western and Southwestern that lie next to it. Its valleys and parks are covered with loess, and are treeless. ”

“ 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. ”

“ Nor will De Candolle's theory, that the limits are governed by the values of heat which are useful to a plant, assist the student; for climatic causes are not the only ones which limit vegetable species, or we should then find the same species growing in every portion of any isothermal belt of a continent, where the same conditions of heat and moisture exist, which is not the case. Some species, apparently very local in their habits and confined to a very limited area, are found many miles farther north, with no intervening stations. ”

“ Here we have a plant, capable of propagating itself in New Jersey, which was long thought to be its only home, reappearing several degrees farther north, where the climate is colder and otherwise different, and yet unknown west of the Alleghany Mountains, where the climate is very like that of New Jersey. ”

“ Allied species are found to be plentiful in the tertiary formation nearly across the continent, indicating that this class of trees at one time reached from the Alleghanies to the Rocky Mountains, the middle of the belt having been destroyed by the more recent changes of the physical conditions of the earth's surface. ”

“ Some of its plants are found farther north; but small beds of alluvial are not uncommon along the coast as far north as New Jersey.
The Campestrian province might be considered as one region, but Cooper [3] has divided it into five. The Saskatchewan region, embracing all north of latitude 49°, together with the basin of the Red River of the North, has some spurs of the Canadian region running into it, and consequently some of the Canadian species are found on them and on the adjacent Silurian and other formations. This region has no characteristic trees of its own. ”

“ But when the great rivers cut their beds down to nearly their present level, the timber gradually died out; not being burned, as some suppose, but disappearing because the geological formation will not retain moisture enough to sustain forest-growth. ”