553 PHYSICAL GEOGRAPHY. 



Marine botany exhibits a similar diversity of species in different temperatures and 

 localities of the ocean. The Polar Atlantic, the West Indian seas, the Indian Ocean 

 and its gulfs, the eastern shores of South America, the coasts of Australasia, the 

 Mediterranean and Red seas, have each peculiar kinds of alga?, or sea-weed, belon"-in<r 

 to them, though some marine productions take a very wide range, and appear to be 

 universally diffused. The genus fucus, which grows up to the surface from deeply sunk 

 rocks, forming immense beds, which act as natural breakwaters, and appear like exten 

 sively inundated meadows, through which ships with difficulty can make their way, is 

 found from the extreme southern islets near Cape Horn, along the whole western coast of 

 South America, and in the tropical Atlantic Ocean. It is abundant in the northerly regions 

 of the deep. This plant has already been mentioned as remarkable for the enormous 

 length of its stems, in some cases reaching to three hundred and sixty feet. It is a 

 striking example, also, of rapid growth. Mr. Stephenson found that a rock uncovered 

 only at spring-tides, which had been chiselled smooth in November, was thickly covered 

 with fucus digitatus two feet in length, and fucus cxculentus six feet, in the following 

 May, within six months afterwards. Some terrestrial plants, likewise, are extensively 

 diffused, and adapt themselves to every variety of climate. The Samolus valerandi, a 

 flowering marsh plant, occurs all over the globe, associated with the birches of the frozen 

 north, and with the palms of the burning tropics. 



But climate only suffices very partially to explain the phenomena of vegetable distribu 

 tion ; for under the same, or corresponding parallels of latitude, at the same elevations 

 above the level of the sea, and upon kindred soils, we find totally distinct genera, and 

 different species. The genus erica, or the common heath, seems to be exclusively confined 

 to one side of our planet, It is chiefly to be found in a narrow longitudinal zone, 

 extending from the northern parts of Europe to the Cape of Good Hope ; for it rarely 

 occurs in Asia, and of the 300 known species only one belongs to America. " Large 

 commons without heaths," says Sir C. Lyell, speaking of his first transatlantic impressions, 

 in the neighbourhood of Boston, "reminded me of the singular fact that no species of 

 heath is indigenous on the American continent." On the other hand, the cactus family 

 belongs as exclusively to the New World as the heaths to the Old. The beautiful and 

 fragrant rose-tree appears to be entirely wanting as a native plant in South America, 

 and throughout the southern hemisphere ; and in general, comparing the vegetation of 

 the two continents, where the same genera recur, the species are not identical. Humboldt 

 found upon the lofty mountains of equinoctial America, where the climate corresponds 

 with that of the temperate zone, plantains, valerians, arenarias, ranunculuses, medlars, 

 oaks, and pines, which, from their physiognomy, might be confounded with those of 

 Europe, but they were all specifically different. The plants of Australasia, with very 

 few exceptions, are different to those of the rest of the world ; and of sixty-one native 

 species, in the little island of St. Helena, only two or three are to be found in any other 

 part of the globe. In some instances, upon travelling across a ridge of mountains, without 

 any change of latitude, the vegetation is found quite different on the one side from the other. 

 Nothing can be more striking than the contrast between the vegetation of the eastern 

 and western sides of the chain of the Rocky Mountains. On the eastern side azaleas, 

 rhododendrons, magnolias, with a variety of oaks and elms, form the principal features 

 of the landscape ; but beyond the ridge, most of these genera entirely disappear, and the 

 giant pine becomes the chief object in view. Starting in an easterly direction through the 

 northern parts of the Old World, we gradually lose the oak, the wild-nut, and the apple- 

 tree, so common in Europe, upon crossing the Uralian mountains, and they cease to be met 

 with beyond the banks of the Tobol ; but in the eastern parts of Asia, on the banks of the 

 Argouan, the two former occur anew, and the last re-appears in the Aleutian isles. 



