Our knowledge of plant anatomy, and of those laws of growth which govern certain classes of plants to-day and in past time, has been very materially widened and extended by the facts revealed to us by the detailed study of Coal-Measure species. The modern science of Plant Biology, refounded by Charles Darwin, has thrown considerable light on the laws of plant life, and it enables us to correlate structural characteristics with physiological conditions of growth. Applying the knowledge gained from living plants to the study of such extinct types as permit of close microscopic examination, we may obtain a glimpse into the secrets of the botanical binomics of Palaeozoic times. The wider questions of climatic conditions depend very largely upon the evidence of fossil botany for a rational solution. As an instance of the best authenticated and most striking alternation in climatic conditions in comparatively recent times, we may cite the glacial period or Ice-Age. The existence of Arctic conditions has been proved by purely geological evidence, but it receives additional confirmation, and derives a wider importance from the testimony of fossil plants. In rocks deposited before the spread of ice from high northern latitudes, we find indubitable proofs of a widely distributed subtropical flora in Central and Northern Europe. Passing from these rocks to more recent beds there are found indications of a fall in temperature, and such northern plants as the dwarf Birch, the Arctic Willow and others reveal the southern extension of Arctic cold to our own latitudes. The distribution of plants in time, that is the range of classes, families, genera and species of plants through the series of strata which make up the crust of the earth, is a matter of primary importance from a botanical as well as from a geological point of view. Among the earlier writers, Brongniart recognised the marked differences between the earlier and later floras, and attempted to correlate the periods of maximum development of certain classes of plants with definite epochs of geological history. He gives the following classification in which are represented the general outlines of plant development from Palaeozoic to Tertiary times[41]. I. Reign of Acrogens ╭ 1. Carboniferous epoch. ╰ 2. Permian epoch. II. Reign of Gymnosperms ╭ 3. Triassic epoch. ╰ 4. Jurassic epoch (including the Wealden). III. Reign of Angiosperms ╭ 5. Cretaceous epoch. ╰ 6. Tertiary epoch.