tribute itself in groups ordered according to the latitude; and this movement once started, the differences between the groups became continually more accentuated by the increasing exclusion in each of them of a part of the types which they originally contained. There resulted from this a constant impoverishment of northern countries as compared with southern ones, which gained, at least by contrast, what the former never ceased to lose. The movement has thus tended to a differentiation by zones, and has resulted in despoiling them, but in unequal proportions, increasingly as they are removed from the tropical zone, the only one which has been exempt from the spoliation.
Let us not forget that, parallel with this movement, working, moreover, with an extreme slowness and in harmony with it, another movement, purely organic and evolutionary, although incited, if not directed, by the former, has not ceased to push to development and to morphological differentiation various groups of plants; particularly of those which, relatively young and plastic, were susceptible, by this fact, of giving birth to new forms, and, by successive splittings, to new types. These are the angiosperms, which, having once gained preponderance, have offered the spectacle of an increasing multiplicity of races and forms. That multiplicity could only increase. The displacements resulting from the climatic depressions have aided in it by inducing changes of stations and opening new cantonments to races not yet fully established. The revolutions of the surface, continental contiguities, and the more or less accentuated orographic relief, have constituted other factors not less active in the general push of species, incessantly solicited to vary as they adapted themselves to the soil of the regions into which they penetrated, as they scattered, and as they struggled victoriously against rival species.
Such is the spectacle which the vegetation of the globe has not ceased to present; and the existing forests appear as the final resultant and ultimate consequence of that long series of alternatives which is summarized in the expression, "the struggle for existence."—Translated for the Popular Science Monthly from the Revue des Deux Mondes.
