One of these, and perhaps the most remarkable of all, is that certain principles were settled very far back, and have remained ever since. We have seen that in the earliest geological periods all that pertains to the structure, powers, and laws of the vegetable cell was already fixed and settled. When we consider how much this implies of mechanical structure and chemical and vital property, the profound significance of this statement becomes apparent. The relations in these respects between the living cell and the soil, the atmosphere and the sunshine, were apparently as perfect in the early Palæozoic as in any subsequent time. The same may be said of the structures of the leaf and of the stem. In such old forms as Nematophyton these were, it is true, peculiar and rudimentary, but in the Devonian and Carboniferous the structure of leaves and stems embodied all the parts and principles that we find at present. In regard to fructification there has been more progress, for, so far as we know, the highest and most complex forms of flowery, fruits, and seeds belong to the more recent periods, and simpler forms were at least dominant in the older times. Yet even in this respect the great leading laws and structures of bisexual reproduction were perfected in the early Palæozoic, and the improvements introduced in the gymnosperm and the angiosperm of later periods have consisted mainly in additions of accessory parts, and in modifications and refinements suited to the wants of the higher and more complex types.
GENERAL LAWS OF ORIGIN AND MIGRATIONS OF PLANTS.--RELATIONS OF RECENT AND FOSSIL FLORAS.
The origination of the successive floras which have occupied the northern hemisphere in geological time, not, as one might at first sight suppose, in the sunny climes 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 the last few years it has produced a number of memoirs and addresses to learned societies, besides many less formal notices.[ED]