316 THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



of the medium in which they live ; and the great uniformity of their 

 organization, with the limited number of their species, indicate an 

 almost constant uniformity of exterior conditions. The ambient medium 

 of the first organisms was incontestably water; and it is admitted that 

 marine plants preceded those of the lagoons, and the latter the vegeta- 

 tion of fresh water. Terrestrial plants followed the aquatic, and are dis- 

 tinguished by a greater variety of forms. The Equisetacea represented 

 to-day by the horse-tail or shave-grass and other analogous plants were 

 the most generally distributed over the islands and continents of the 

 first periods. They appear in all the sedimentary strata, from the most 

 ancient to the most recent. The number of their fossil species known 

 exceeds a hundred. The conformation of their stem permits their 

 division into three categories, which mark the periods of development. 

 The Equisetacea of the first period up to the Graywacke are arborescent 

 calamites, whose longitudinal grooves continue in an unbroken line to 

 the articulation of the branches. The second period, from the Coal 

 epoch to the Mixed Sandstone, also contains arborescent calamites, 

 but their grooves alternate with the articulations and the line of sepa- 

 ration is broken in the form of a zigzag. Certain paleontologists explain 

 why the sheath is wanting in the two forms preceding, while it is found 

 with the Equisetacea, by supposing that in these two forms only the 

 mold of the interior of the arborescent calamites has been preserved. 



The Lycopodacea, also very ancient, hold a very important place, par- 

 ticularly in the Coal period. The arborescent kinds, such as Lepido- 

 drendon, Lycopodites, Knorria, Segenaria, were giants compared with 

 their humble representatives of the present day, of which the small 

 number of species indicates a rapid decline. No other family of plants 

 is found in the fossil state under so many forms or in such great num- 

 ber. Their exceptional preservation cannot be attributed to the lig- 

 neous and fibrous structure of their stem, but to their actual predomi- 

 nance in the flora of the Coal period. 



The ferns commence with the Graywacke, and are already very 

 abundant in the Old Eed Sandstone. Their fossil remains are often 

 so well preserved that the entire plant can be reconstructed with the 

 greatest certainty, and its character studied as closely as with a living 

 plant; for the most delicate parts, among others the organs of fructifica- 

 tion, have well resisted the action of time, and they furnish very valua- 

 ble data for phytotomy and the comparative organography of fossil and 

 recent ferns. 



Certain remains of fossil plants for a long time puzzled the paleontol- 

 ogists; now it is admitted that the Aster ophyUites, the Stigmarice, and the 

 Sigillarice are only roots and stems of the same species of vegetation, or 

 at least of the same genus. 



In the succession of the classes of the vegetable kingdom the Monoco- 

 tyledons come after the Acotylcdons. The number of their species increase 

 up to the Chalk, and, in all, a hundred fossil species are known, thirty- 



