In Cambrian times, then, abundant life springs forth into our vision from the rocks, already, like Minerva, fully armed. The soft plant structures are not well preserved in the older fossiliferous rocks, and hence the fragmentary story of plant life, as we trace it backwards, becomes very obscure, while many types of animals still boldly occupy the stage. At the earliest period from which plant remains are well preserved and plentiful, in the Devonian rocks, many of the great plant groups are fully developed, the vegetation displaying an abundance and luxuriance comparable to that of the present day. Seaweeds (Algæ), Horsetails (Equisetales), Ferns (Filicales), Club-mosses (Lycopodiales), fill the waters or clothe the land, and Seed Plants are already abundant in the form of the fern-like Pteridosperms, long since extinct. Both as regards adaptation to environment and internal structure a very high degree of specialization has already been obtained. “If a botanist,” writes D. H. Scott, “were set to examine, without prejudice, the structure of those Devonian plants which have come down to us in a fit state for such investigation, it would probably never occur to him that they were any simpler than plants of the present day; he would find them different in many ways, but about on the same general level of organization.”
In the succeeding Carboniferous Period conditions appear to have been peculiarly suitable for vegetable life, as well as for its preservation in a fossil condition. In the warm, moist climate of those times, many of the races of plants above mentioned attained an imposing size, luxuriance, abundance, and variety; and their remains, fortunately well preserved owing to conditions favourable to slow decomposition, not only furnish a rich heritage for the botanist, but supply the coal, on the energy derived from which our whole modern civilization is built up.
Before the end of the Palæozoic Period the Conifers had appeared, descended possibly from the extinct _Cordaiteæ_. With the advent of the Secondary or Mesozoic epoch the group of the Cycads, to which our modern Screw-pines belong, rose to great importance, descended probably from the Pteridosperms, and long continued to be a dominant feature of terrestrial vegetation. And then at last in the Lower Cretaceous rocks the Angiosperms, or “Flowering Plants” _par excellence_, both Dicotyledons and Monocotyledons, put in an appearance. It seems probable that they were evolved from Cycads, such as the _Bennettiteæ_, recent researches on magnificent fossil material discovered in America showing striking analogies between certain Cycadaceous flowers and those of such plants as Magnolias, Water Lilies, and Buttercups. Once established, the Angiosperms rose to primary importance in an extraordinarily short time--very possibly owing to the “invention” of insect pollination, which may have arisen at that period. In Upper Cretaceous times the two great groups into which the Angiosperms still fall, the Dicotyledons and Monocotyledons, fairly dominated the flora of the world, as they do at present. Already many types familiar at the present day had appeared, and the woods were filled with Birches, Beeches, Oaks, Planes, Maples, Hollies, Ivies, as they are nowadays.