58 
INFLUENCE OF CULTIVATED PLANTS. 
tive, vegetables, and even wild beasts, restorative powers. 
The rushing waters sweep down earth from the uplands ; in 
the first moment of repose, vegetation seeks to reestablish 
itself on the bared surface, and, by the slow deposit of its 
decaying products, to raise again the soil which the torrent 
had lowered. So important an element of reconstruction is 
this, that it has been seriously questioned whether, upon the 
whole, vegetation does not contribute as much to elevate, as 
the waters to depress, the level of the surface. 
Whenever man has transported a plant from its native 
habitat to a new soil, he has introduced a new geographical 
force to act upon it, and this generally at the expense of some 
indigenous growth which the foreign vegetable has supplanted. * 
The new and the old plants are rarely the equivalents of each 
other, and the substitution of an exotic for a native tree, shrub, 
or grass, increases or diminishes the relative importance of the 
vegetable element in the geography of the country to which 
it is removed. Further, man sows that he may reap. The 
products of agricultural industry are not suffered to rot upon 
the ground, and thus raise it by an annual stratum of new 
mould. They are gathered, transported to greater or less dis¬ 
tances, and after they have served their uses in human econ¬ 
omy, they enter, on the final decomposition of their elements, 
into new combinations, and are only in small proportion 
returned to the soil on which they grew. The roots of the 
grasses, and of many other cultivated plants, however, usually 
remain and decay in the earth, and contribute to raise its 
surface, though certainly not in the same degree as the forest. 
The vegetables, which have taken the place of trees, 
unquestionably perform many of the same functions. They 
radiate heat, they condense the humidity of the atmosphere, 
they act upon the chemical constitution of the air, their roots 
penetrate the earth to greater depths than is commonly sup¬ 
posed, and form an inextricable labyrinth of filaments which 
bind the soil together and prevent its erosion by water. The 
broad-leaved annuals and perennials, too, shade the ground, 
and prevent the evaporation of moisture from its surface by 
