We have said something of what was in the Palæozoic flora; but what of that which was not? We may answer:—Nearly all that is characteristic of our modern forests, whether in the ordinary Exogens, which predominate so greatly in the trees and shrubs of temperate climates, or in the Palms and their allies, which figure so conspicuously within the tropics. The few rare, and to some extent doubtful, representatives of these types scarcely deserve to be noted as exceptions. Had a botanist searched the Palæozoic forests for precursors of the future, he would probably have found only a few rare species, while he would have seen all around him the giant forms and peculiar and monotonous foliage of tribes now degraded in magnitude and structure, and of small account in the system of nature. It must not be supposed that the Palæozoic flora remained in undisturbed possession of the continents during the whole of that long period. In the successive subsidences of the continental plateaux, in which the marine limestones were deposited, it was to a great extent swept away, or was restricted to limited insular areas, and these more especially in the far north, so that on re-elevation of the land it was always peopled with northern plants. Thus there were alternate restrictions and expansions of vegetation, and the latter were always signalised by the introduction of new species, for here, as elsewhere, it was not struggle, but opportunity, that favoured improvement. In the Lower Silurian such plants as existed must have experienced great restriction at the age of the Niagara or Wenlock limestone. Those of the Upper Silurian suffered a similar reverse at the time of the Lower Helderberg or Ludlow limestones. This recurred at the close of the Devonian and in the time of the Lower Carboniferous limestone; and finally the Palæozoic flora disappeared altogether in the Permian, to be replaced by new types in the Mesozoic. While, therefore, there is a great general similarity in the successive Palæozoic floras, there are minor differences, so that the Devonian plants are for the most part distinct specifically from those of the Lower Carboniferous, those of the Lower Carboniferous from those of the Coal-formation, and those of the latter from those of the Permian.