26 THE GEOLOGICAL HISTORY OF PLANTS. 



gard the supposed stigmaria-like roots as really stems, 

 and the supposed rootlets as short, spine-like rudiment- 

 ary leaves. All such comparisons must, however, in the 

 mean time be regarded as conjectural. We seem, how- 

 ever, to have here a type of tree very dissimilar to any 

 even of the later Palaeozoic age, which existed through- 

 out the Silurian, and probably further back, which ceased 

 to exist early in the Erian age, and before the appearance 

 of the ordinary coniferous and lepidodendroid trees. 

 May it not have been a survivor of an old arboreal flora 

 extending back even to the Laurentian itself ? 



Multitudes of markings occurring on the surfaces of 

 the older rocks have been referred to the Algae or sea- 

 weeds, and indeed this group has been a sort of refuge for 

 the destitute to which palaeontologists have been accus- 

 tomed to refer any anomalous or inexplicable form which, 

 while probably organic, could not be definitely referred to 

 the animal kingdom. There can be no question that some 

 of these are truly marine plants ; and that plants of this 

 kind occur in formations older than those in which we first 

 find land-plants, and that they have continued to inhabit 

 the sea down to the present time. It is also true that the 

 oldest of these Algae closely resemble in form plants of 

 this kind still existing ; and, since their simple cellular 

 structures and soft tissues are scarcely ever preserved, 

 their general forms are all that we can know, so that their 

 exact resemblance to or difference from modern types can 

 rarely be determined. For the same reasons it has proved 

 difficult clearly to distinguish them from mere inorganic 

 markings or the traces of animals, and the greatest di- 

 vergence of opinion has occurred in recent times on these 

 subjects, as any one can readily understand who consults 

 the voluminous and well-illustrated memoirs of Nathorst, 

 Williamson, Saporta, and Delgado. 



The author of this work has given much attention to 

 these remains, and has not been disposed to claim for the 



