[85] Cordaites, etc. As I have elsewhere shown, these are distinct sub-floras in the Lower, Middle and Upper Devonian, and in the Lower, Middle and Upper Carboniferous and Permian, sufficiently different to allow these periods to be determined by the evidence of these fossil plants. Reports prepared for Geological Survey of Canada.
The early Mesozoic is altogether peculiar. It shows a vast predominance of Cycads, Pines and Ferns, to the exclusion both of the gigantic Cryptogams of the Palæozoic and of the ordinary exogenous trees of the modern time. It has a strange, weird aspect, and more resembles that of some warm islands of the southern hemisphere at present, than anything else known to us. It is as if the flora of some southern island had migrated and invaded all parts of the world. The geographical and climated conditions which permitted this must have been of a character different from those both of earlier and later times.
As we approach to the termination of the Mesozoic, which, in regard to animal life, is the age of reptiles, a new and strange development meets us. We find beds filled with leaves of broad-leaved plants similar to those of our modern woods, and in most cases apparently belonging to the same genera with plants now living, and this new type of vegetation persists to the present, though with marked differences of species in successive eras, as in the Middle and Upper Cretaceous, and the Lower, Middle and Upper Kainozoic, or Tertiary. It is noteworthy that while this new vegetation not only altogether supersedes the great Cryptogamous forests of the Palæozoic, but replaces the Cycads of the immediately preceding eras, the Pines retain all their prominence and grandeur, and even seem to excel in number of species, in breadth of dispersion, and in magnitude of growth their successors in the present world.
While in the latter Cretaceous and Early Tertiary, the northern hemisphere at least seems to have enjoyed an exceptionally warm climate, the later Tertiary introduces that period of cold known as the Glacial age. While there is no doubt that the intensity of this glaciation has been greatly exaggerated by extreme glacialists, and while it is certain that some vegetation, and this not altogether of Arctic types, continued to exist throughout this period, even in the now temperate regions of our continents, it is evident that a great reduction of the exuberance of the flora occurred by the removal of many species, and that the present flora of the northern hemisphere is inferior in variety and magnificence to that of the Middle Tertiary, just as it is found that the Mammalian fauna of our continents has since that time been reduced both in the number and magnitude of its species.
If the reader has followed this general sketch, he will be prepared to appreciate some examples of a more detailed character relating to the floras of different periods, and some discussions of general points relating to the genesis and vicissitudes of the vegetable kingdom.
The origination of the more important floras which have occupied the northern hemisphere in geological times, not, as one might at first sight suppose, in the sunny climates of the South, but under the arctic skies, is a fact long known or suspected. It is proved by the occurrence of fossil plants in Greenland, in Spitzbergen, and in Grinnell Land, under circumstances which show that these were their primal homes. The fact bristles with physical difficulties, yet is fertile of the most interesting theoretical deductions, to reach which we may well be content to wade through some intricate questions. Though not at all a new fact, its full significance seems only recently to have dawned on the minds of geologists, and within recent years it has produced a number of memoirs and addresses to learned societies, besides many less formal notices.[86]
[86] Saporata, "Ancienne Vegetation Polaire"; Hooker, Presidential Address to Royal Society, 1878; Thistleton Dyer, "Lecture on Plant Distribution "; Mr. Starkie Gardner, Letters in Nature, 1878, etc. The basis of most of these brochures is to be found in Heer's "Flora Fossilis Arctica."
The earliest suggestion on this subject known to the writer is that of my old and dear friend, Professor Asa Gray, in 1867, with reference to the probable northern source of the related floras of North America and Eastern Asia. With the aid of new facts disclosed by Heer and Lesquereux, Gray returned to the subject in 1872, and more fully developed this conclusion with reference to the Tertiary floras,[87] and still later he further discussed these questions in an able lecture on "Forest Geography and Archæology."[88] In this he puts the case so well and tersely that I may quote the following sentences as a text for what follows:—
[87] Address to American Association.