FERNS, FERN ALLIES, AND FOSSILS. 283 



crabs, the tubes formed by sandworms, and the 

 cracks of a drying clay surface (not to speak of 

 oyster shells), have been described as algae. But 

 if such a specialised type as the diatom is found in 

 the Cretaceous period, and if Bacteria (on the authority 

 of Renault), existed in the coal-measures, it seems 

 at least probable, that algae must have flourished 

 long before either of these epochs. 



Before proceeding further with the geological history, 

 it is interesting to see how vegetation develops on 

 bare places in the present day. It must not be 

 forgotten that the land has been very greatly altered 

 by vegetation. The plants of the early geological 

 periods had to develop on absolutely unworked soil. 

 There were no surface moulds ; no shales full of 

 organic remains ; but only barren igneous rocks, or 

 limestones, and sedimentary strata scarcely altered by 

 vegetable action. 



At present, it is only new lava flows, such as those 

 from Mt. Pelee and La Soufriere, or from Mt. Vesuvius, 

 and, in a less degree, newly built stone walls and 

 the screes or rockfalls of the highlands that can 

 be compared to primitive conditions. It so happens, 

 that an excellent description has been given by 

 Comes of the gradual settlement by plants of the 

 Vesuvian lavas. The first species noticed were Algae 

 of the Pleurococcus group, and certain Lichens.^ These 

 slightly disintegrated the lava, and by their decay 

 introduced a slight percentage of organic matter. 

 Mosses of different kinds next appeared upon this 

 slightly prepared surface. Then came certain Ferns, 

 and, still later on, Flowering plants, adapted to very 



^ Heer's Primaeval Flora of Switzerland gives an excellent and vivid 

 picture of the succession of plants in central Europe. 



