In fine, from all the genera and species of plants that have been detected in the strata of the coal-measures, it would appear that the flora of that ancient period was in a high degree _acrogenous_--that is to say, consisted in great measure of ferns, club-mosses, and other members of the great group of plants known as _acrogens_. This word literally means _top-growers_, and is applied to those plants which increase in height, but not in width, since they attain at first nearly their ultimate diameter. Such plants occupy a low position in the botanical scale. Mingled with the numerous genera of carboniferous ferns and club-mosses, we find the remains of a much higher grade of vegetation--that of the _gymnogens_, or plants that bear naked seeds--such as the firs and pines. There also seem to have been a few _endogenous_ flowering plants. Viewing, then, this flora on the whole, it presents us with many striking resemblances to certain botanical regions of the present day. Many of the tropical islands abound in ferns, and contain very few flowering plants. But New Zealand affords perhaps the closest parallel. That island is in certain parts highly mountainous, its loftiest summits being covered with glaciers. The hills throughout large districts are bare, or covered with a scanty herbage, while in other localities they are densely clothed with forests of pine, beech, and other trees. These forests sweep on to the lower grounds, where they are replaced by a thick growth of fern and flax-plant intermingled with dragon-trees and graceful tree-ferns, while the more swampy regions support a rich profusion of reeds and rushes. Such a condition of things affords a close parallel to the probable vegetation of the Carboniferous period--an immense preponderance of ferns and arborescent acrogens, with an intermixture of large coniferous trees. From the general scantiness of a flora where ferns predominate, it has been argued that the swamps of the coal-measures nourished a luxuriant repetition of comparatively few species; and this hypothesis also receives confirmation from the vegetation of New Zealand. Another deduction founded on the resemblance of the ancient to the modern flora, refers to the conditions of heat and moisture. It has been inferred that the climate of the coal period was equable and humid, like that of New Zealand--a supposition much more natural and simple than that, once so much in vogue, of a heated atmosphere densely charged with carbonic acid gas. That the air of the Carboniferous period differed in no material respect from the air of the present day, seems at last proved by the remains of air-breathing animals having been found among the coal-beds; and there seems no reason why the higher mountain-tops of the same epoch may not have been clothed with glaciers as those of New Zealand are. As yet we have no evidence of the fact, but it is by no means beyond the possibility of proof.[29]