Above sixteen different forms of vegetation are principally concerned in determining the aspect or physiognomy of Nature. I mention only those which I have observed in the course of my travels both in the New and Old Continents, where during many years I have attentively examined the vegetation of the regions comprised between the 60th degree of North and the 12th degree of South latitude. The number of these forms will no doubt be considerably augmented when travellers shall have penetrated farther into the interior of Continents, and discovered new genera of plants. In the South-eastern part of Asia, the interior of Africa and of New Holland, and in South America from the river of the Amazons to the province of Chiquitos, the vegetation is still entirely unknown to us. How if at some future time a country should be discovered in which ligneous fungi, Cenomyce rangiferina, or mosses, should form tall trees? The Neckera dendroides, a German species of moss, is in fact arborescent; and bamboos (which are arborescent grasses) and the tree ferns of the tropics, which are often higher than our lime-trees and alders, now present to the European a sight as surprising as would be that of a forest of tree mosses to its discoverer. The absolute size and the degree of development attained by organic forms of the same family (whether plants or animals), depend on laws which are still unknown to us. In each of the great divisions of the animal kingdom, insects, crustacea, reptiles, birds, fishes, or mammalia, the size of the body oscillates between certain extreme limits. But these limits, which have been established by observation as far as it has yet gone, may be corrected by the discovery of species with which we are still unacquainted.