above-mentioned types (Walchia) may indeed be regarded as allies of the modern Araucarian pines of the southern hemisphere, and of the old conifers of the Carboniferous. Those of the second type (Ulmannia) may be referred to the same group with the magnificent Sequoias or Redwoods of California. [X] Walchia. [Y] Ulmannia. It is a curious indication of the doubts which sometimes rest on fossil botany, that some of the branches of these Permian pines, when imperfectly preserved, have been described as sea-weeds, while others have been regarded as club-mosses. It is true, however, that the resemblance of some of them to the latter class of plants is very great; and were there no older pines, we might be pardoned for imagining in the Permian a transition from club-mosses to pines. Unfortunately, however, we have pines nearly as far back in geological time as we have club-mosses; and, in so far as we know, no more like the latter then are the pines of the Permian, so that this connection fails us. In all probability the Permian forests are much less perfectly known to us then those of the coal period, so that we can scarcely make comparisons. It appears certain, however, that the Permian plants are much more closely related to the coal plants then to those of the next succeeding epoch, and that they are not so much a transition from the one to the other as the finishing of the older period to make way for the newer. But we must reserve some space for a few remarks on the progress and termination of the Palæozoic as a whole, and on the place which it occupies in the world's history. These remarks we may group around the central question, What is the meaning or value of an age or period in the history of the earth, as these terms are understood by geologists? In most geological books terms referring to time are employed very loosely. Period, epoch, age, system, series, formation, and similar terms, are used or abused in a manner which only the indefiniteness of our conceptions can excuse.