Since Brongniart’s time this method of classification has been extended to many of the smaller subdivisions of the geological epochs, and species of fossil plants are often of the greatest value in questions of correlation. In recent years the systematic treatment of Coal-Measure and other plants in the hands of various Continental and English writers has clearly demonstrated their capabilities for the purpose of subdividing a series of strata into stages and zones[42]. The more complete becomes our knowledge of any flora, the greater possibility there is of making use of the plants as indices of geological age[43]. [Sidenote: FOSSIL PLANTS AND PHYLOGENY.] Not only is it possible to derive valuable aid in the correlation of strata from the facts of plant distribution, but we may often follow the various stages in the history of a particular genus as we trace the records of its occurrence through the geologic series. In studying the march of plant life through past ages, the botanist may sometimes follow the progress of a genus from its first appearance, through the time of maximum development, to its decline or extinction. In the Palaeozoic forests there was perhaps no more conspicuous or common tree than the genus long known under the name of _Calamites_. This plant attained a height of fifty or a hundred feet, with a proportionate girth, and increased in thickness in a manner precisely similar to that in which our forest trees grow in diameter. The exceptionally favourable conditions under which specimens of calamitean plants have been preserved, have enabled us to become almost as familiar with the minute structure of their stems and roots, as well as with their spore-producing organs, as with those of a living species. In short, it is thoroughly established that _Calamites_ agrees in most essential respects with our well known _Equisetum_, and must be included in the same order, or at least sub-class, as the recent genus of _Equisetaceae_. As we ascend the geologic series from the Coal-Measures, a marked numerical decline of _Calamites_ is obvious in the Permian period, and in the red sandstones of the Vosges, which belong to the same series of rocks as the Triassic strata of the Cheshire plain, the true _Calamites_ is replaced by a large _Equisetum_ apparently identical in external appearance and habit of growth with the species living to-day. In the more recent strata the Horse-tails are still represented, but the size of the Tertiary species agrees more closely with the comparatively small forms which have such a wide geographical distribution at the present time. Thus we are able to trace out the history of a recent genus of Vascular Cryptogams, and to follow a particular type of organisation from the time of its maximum development, through its gradual transition to those structural characters which are represented in the living descendants of the arborescent Calamites of the coal-period forests. The pages of such a