5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 133 



be that their affinity is with the Algae, and it even seems conceivable 

 that they may represent the long-sought link between that group 

 and the lowest land-plants. A further link may be forged by 

 certain cellular forms producing firm- walled spores, which forms, 

 like some Nematophytales, have a covering cuticle resembling that of 

 land- plants, but which seem to belong to a relatively advanced group 

 without actually being typical land-plants in other respects. 



P'ossil remains of Bryophyta are not common, probably owing to 

 the fragile nature of the plant body. Some Liverworts are known 

 from as early as the Carboniferous towards the close of the Palaeozoic 

 era ; traces of Mosses have also been found in rocks laid down before 

 the end of the Carboniferous. From the Triassic period onwards, 

 fossil Bryophytes tend to become less rare, so that a considerable 

 number are known from the Pleistocene. It now seems that fossil 

 Bryophytes throw little if any light on the problem of the origin of 

 vascular plants, and that there is no justification for thinking them 

 ever to have served as intermediate stages in the evolution of 

 higher plants. Instead, these presumably evolved from Algae, as 

 also with little doubt did the Brvophvtes — but in this latter instance 

 without going much ahead. Consequently they are little changed 

 to this day, although they are now numerous and often ecologically 

 successful within the limits prescribed by their relative diminutive- 

 ness. 



By the close of the Silurian period there were undoubted land- 

 plants, the primitive aquatic or semi-aquatic Algae (perhaps through 

 more advanced types such as the Nematophytales or others of which 

 we have no knowledge) having apparentlv come out on land and 

 given rise to vascular forms. As we saw in Chapter II, these last 

 are characterized by the possession of a conducting system com- 

 posed essentially of wood and bast elements ; nor is it by any means 

 impossible that such a system was developed among marine Thallo- 

 phytes, the more adaptable of which may gradually have become 

 transformed to withstand permanent life on land. At the same time 

 their holdfasts could have developed into rhizome-like structures 

 bearing rhizoids. For such are the earliest known land-plants, and 

 so may the great ' subaerial transmigration ' have taken place. 



The earliest known plants that were clearly adapted to life on land 

 belong to the class Psilophytineae, which was probably more primitive 

 than any of the other Pteridophytes, and of which some reconstructed 

 examples are show^n in Fig. 30, A and B. They range from the 

 middle Silurian to the upper Devonian periods of the Palaeozoic era, 



