"In the existing climates of the earth we find these classes of plants variously distributed as to relative numbers. In some, pines predominate. In others, palms and tree-ferns form a considerable part of the forest vegetation. In others, the ordinary exogenous trees predominate, almost to the exclusion of others. In some Arctic and Alpine regions, mosses and lichens prevail. In the Coal period we have found none of the higher Exogens, though one species is known in the Devonian, and only a few obscure indications of the presence of Endogens; but Gymnosperms abound, and are highly characteristic. On the other hand, we have no mosses or lichens, and very few algæ, but a great number of ferns and Lycopodiaceæ or club-mosses. Thus the coal formation period is botanically a meeting-place of the lower Phænogams and the higher Cryptogams, and presents many forms which, when imperfectly known, have puzzled botanists in regard to their position in one or other series. In the present world, the flora most akin to that of the Coal period is that of moist and warm islands in the southern hemisphere. It is not properly a tropical flora, nor is it the flora of a cold region, but rather indicative of a moist and equable climate. In accordance with this is the fact that the equable but not warm climate of the southern hemisphere at present (which is owing principally to its small extent of land) enables sub-tropical plants to extend into high latitudes. In the Coal period this uniformity was evidently still more marked, since we find similar plants extending from regions within the Arctic circle to others near to the tropics. Still we must bear in mind that we may often be mistaken in reasoning as to the temperature required by extinct species of plants differing from those now in existence. Further, we must not assume that the climatal conditions of the northern hemisphere were in the Coal period at all similar to those which now prevail. As Sir Charles Lyell has argued, a less amount of land in the higher latitudes would greatly modify climates, and there is every reason to believe that in the Coal period there was less land then now. It has been shown by Tyndall that a very small additional amount of carbonic acid in the atmosphere would, by obstructing the radiation of heat from the earth, produce almost the effect of a glass roof or conservatory, extending over the whole world. There is much in the structure of the leaves of the coal plants, as well as in the vast amount of carbon which they accumulated in the form of coal, and the characteristics of the animal life of the period, to indicate, on independent grounds, that the Carboniferous atmosphere differed from that of the present world in this way, or in the presence of more carbonic acid--a substance now existing in the very minute proportion of one-thousandth of the whole by weight, a quantity adapted to the present requirements of vegetable and animal life, but probably not to