Changes insensible at each single step have in the long run resulted in modifications of the aspect of the landscape and replacements of the types and species composing the floral carpet at a given movement by forms different from them, and also from those before which they were themselves destined to retire at a later period.
The impression which one feels in the midst of a deep forest is one of perennial duration. Except man, what is there to uproot those giants that have lived through centuries? What action can be conceived of that will exclude them from the ground which they possess so completely? The first impression would almost make these forest masses coeval with the globe, its natural product and spontaneous dress from the days of its youth. Such an impression would be a mistaken one. The forests have not been perpetuated in the same order from the beginning, but have changed much in the course of ages. Those which we now see have taken the place of other more ancient ones, and these substitutions have occurred many times, sometimes through partial modifications and sometimes also under such conditions that the old order has only indirect relations with the present one, or is even wholly foreign to it.
Since a serious mind can not suppose that at every revolution of plant-life there has been a total destruction of the anterior elements, followed by a creation conceived anew in all its details, we are forced to seek in the order which precedes the reason for the existence of that which has replaced it. This view implies an endless chain of causes and effects, of ancestral and derived forms, stretching along, now spreading, now continuing themselves, to spread out again, and—in what more particularly concerns the types of the vegetable kingdom—emigrating in a determined direction. This direction is found to have consisted, for plants, in a march from north to south in search of more favorable regions and stations better fitted to the exigencies of acquired adaptation, as rapidly as the terrestrial temperature declined from its pristine conditions, as latitudes took on their individual characteristics, and as the arctic zone, which had been temperate, grew cooler and became more and more differentiated. The polar circle was thus constituted a barrier that became more pronounced, less accessible, and was finally closed to arboreal vegetation, while under the operation of the same movement the present temperate zone became cooled in an equivalent measure, was impoverished, and gradually stripped of a considerable part of its floral wealth. The remnants that escaped this elimination in those successive and numerous retreats that filled the second half of the Tertiary period still occur, scattered and dwarfed, in the southern part of that zone, and upon points where the less sensible depression of
