The virgin forest, moreover, is one of the sanctuaries of Nature, where her mysteries are seldom profaned by man. There life reveals itself, and moves at liberty, under an infinite variety of forms. It is the asylum of a multitude of animals of all classes, which find therein, united, the two essential conditions of existence--shelter and nourishment. Without the difficult approaches, the obscurity and the profound depth of the forests, says a naturalist, what would become of the species of mammals, birds, and reptiles, against which man wages incessant war? Nature, then, seems to have provided these immense reservoirs to prevent their species from being totally annihilated. Independently of the trees which constitute the forests, a host of other plants make them their exclusive _habitat_; thence the specific and eminently characteristic names--such as _Sylvestris_, _Sylvaticus_, _Nemorosus_--imposed upon a great number among them. Such plants are distinguished from their congeners by the great dimensions of their stems; but, on the other hand, they do not possess the brilliantly-coloured flowers which adorn the plants of the mountains and the plains always exposed to the action of the solar light. The forests, moreover, offer for the botanist this remarkable and singularly precious circumstance, that they form natural collections of trees of the same species, or of several species of the same genus, or at least of the same family; so that their limits circumscribe the _habitat_ of these grand vegetables, and permit us to determine with ease their geographical distribution. [Illustration: THE VIRGIN FOREST OF THE GABOON.] The forests fill an important function in the general economy of the globe, by the influence which they exercise upon the mean temperature and the other meteorological conditions of the regions they shelter. All other things being equal, the temperature of well-wooded countries is perceptibly less elevated and more uniform than that of dry and open districts. The amount of humidity which is retained on the surface of the soil by wide-spread woods is considerable; it results from the lesser evaporation of the waters, the abundant transpiration of the leaves, and the heavy rains which inundate the forests during the tropical summer. Forests, like mountains, seem to attract the clouds. So the plains which lie on their borders are ever better watered and fertile than those whose horizon no obstacle encumbers.