612 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



phytes which may have existed on land, but we have huge tree-like Vascular 

 Cryptogams with trunks, fronds, and leaves which are to be placed side by side 

 with our present-day Bquisetums, Ferns, and Lycopods. Cycads and Conifers 

 also are not absent from the Coal Measures. No Angiospermous flowering plants 

 have hitherto been demonstrated in these strata, but it would be foolish to 

 regard this fact as a proof that neither Thallophytes nor Angiosperms flourished 

 at this period. That which has been preserved to us from this time certainly 

 forms but a small fraction of the old vegetation, and is restricted apparently to 

 the flora of peat-moors which were just as poor in species and just as monotonous 

 as they are to-day. The plants which at the present time predominate on the 

 moors are still the Equisetums, Ferns, Lycopods, and Conifers, and, in tropical 

 regions, the Cycads; only a few species from each group, but standing in thousands 

 side by side and aggregated into dense communities. Anyone who has worked 

 out the history of these moors knows that the soil must have been prepared 

 for these plants by other growths. Equisetum limosum, Aspidium Thelypteris, 

 Lycopodium inundatum, &c., do not flourish in soil poor in humus; in order 

 to obtain their requisite food and to develop they require soil which is saturated 

 with the dead remains of earlier settlers. Experience tells us that the plants 

 which appear as the first inhabitants belong to widely difierent groups (see vol. i. 

 p. 268). Now if we hold to the view that the formation of peat-moors in long- 

 past ages occurred just as in the present day, we must assume that the colonies 

 of Equisetums, Ferns, Lycopods, and Cycads were preceded by other plants which, 

 as the first settlers, prepared the soil. We cannot indeed determine from the 

 surviving remains to which groups these first settlers belonged; but, looking back 

 on the history of our present peat-moors, it seems not improbable that among 

 them were both Thallophytes and Angiospermous flowering plants. 



The fact that the fossil remains of Equisetums, Lycopods, and Cycads, which 

 spread so widely over the peat-moors of palaeozoic times, have reached us in such 

 good condition is explained by the presence of humus-acids, which are formed 

 universally in the peat (see vol. i. p. 263). There are four conditions which render 

 it possible for a plant to be preserved as a fossil: humus-acids form the first; the 

 second is the resin which exudes from the pine-wood and forms amber; the third is 

 mud and sand brought by floods; and the fourtb the silicification and calcination of 

 the cell-wall or the formation of a lime incrustation which is precipitated from 

 calcareous water on to the various parts of the plant. It is certain that these four 

 conditions have always been efiective, but it is doubtful whether all the fossils 

 formed in the fourth manner at all periods have remained. For many older strata 

 have long been destroyed and used in the building up of younger layers, and many 

 risings and sinkings of these strata have taken place. It would indeed be difficult 

 to find a single place on the earth's surface which has not been repeatedly above 

 and under the sea. Much that might lead us to definite conclusions at present lies 

 inaccessible to us, covered with immense masses of water at the bottom of the sea, 

 and the view has actually been suggested from studies made on the few accessible 



