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.