Page:Popular Science Monthly Volume 34.djvu/251

Rh, distinguished by shades which were gradually effaced on approaching modern times.

The principal requisite for tracing the presumed origins, whether of the type at its birth, of supposed ancestors, or of the direct antecedents of existing species, is to have in mind the exact succession of the periods and stages, giving the relative date of each of the determined first appearances and the order of the constitutive elements of the march which the forms of which we meet the traces have followed through time and space. The succession of ages, represented by beds deposited in a constant order, or stratigraphic geology, makes this known to us. It is also necessary to take account of the changes that have been impressed upon the whole vegetable kingdom during this long series of periods. The changes have been too profound, and attended by too complete renovations, for us to be able to find the cradles of existing plants in the primitive periods.

Three great plant-periods may be distinguished, starting from the moment when the surface of the globe first began to be covered with aerial vegetation: the primary or paleophytic period, or cryptogamic era, which derives its name from the domination of cryptogams; the secondary period, or mesophytic, during which gymnosperms—conifers and cycads—obtained the predominance, while foliage-trees were still absent; and the tertiary or neophytic period, also called angiospermic, from the presence of the higher plants and particularly of the foliage-trees.

The last of these periods, which began with the chalk and is still in continuance, is characterized by the appearance and extension of the higher plants, and was also coincident with the first signs of polar refrigeration and with the more and more marked decrease of terrestrial temperature with increase of latitude. This fact, at first hardly sensible, then gradually accented, exercised an influence within the polar circle before extending its action beyond, into the temperate zone, which was for a long time hot, and afterward warm, while the regions around the pole were already frozen. A remarkable relation certainly seems to exist between the beginning and the course of the climatological depression of the northern regions and the progress of vegetation, which perfected itself in a parallel line. Indeed, it closed the entire cycle of its definitive evolution by the adjunction of the angiosperms, the most perfect plants; and these acquired preponderance as rapidly as the cooling of the arctic regions went on. These regions appear to have been exempt till this time from the rigors of a cold season, and, by that fact, subtracted from the effects of the winter rest.

It is certain that the vegetable kingdom, as soon as it had acquired all the elements of which it is still composed, began to