viii PREFACE 



who will find here, in a small compass, an excellent comparative 

 description of ancient fossils, often little known, the literature 

 on which is much scattered and not always readily accessible. 

 So far as I know, nothing of the kind has been attempted before, 

 for the Devonian Floras, and I can testify to the great utility 

 of the author's critical summary. 



Attention may be specially called to the evidence, which the 

 author adduces, in favour of the generic identity of Psilophyton 

 and Rhynia, an important conclusion, which will have to be 

 taken into serious consideration. On the other hand the remarks 

 on Halle's genus Sporogonites, would undoubtedly have been 

 modified, if the author could have been acquainted with the 

 later evidence; this point is dealt with in a footnote on p. 45. 



Chapter IV, "A Discussion of the Nature and Affinities of 

 the Psilophyton Flora," is of great theoretical importance. The 

 author expresses his conviction that Psilophyton and all the 

 other genera of that Flora "were much more probably Thallo- 

 phyta than Pteridophyta " (p. 47). But he also points out that 

 "Psilophyton,... while still Thallophytic in habit, may occupy 

 anatomically a place -half-way between the Thallophyta and 

 Pteridophyta" (p. 49). This was written early in 1918; it agrees 

 very nearly with the statement by Kidston and Lang (Part n, 

 1920, p. 622) that "The facts are consistent with the Rhyniaceae 

 finding their place near the beginning of a current of change 

 from an Alga-like type of plant to the type of the simpler 

 Vascular Cryptogams." Arber had grasped the position at a 

 time when only a portion of the evidence was before him. 



Chapter VI, "The Procormophyta and the Origin of the Cor- 

 mophyta," completes the exposition of the author's theory. He 

 holds that there were three distinct main lines of descent among 

 vascular plants the Sphenopsida, Pteropsida and Lycopsida, 

 derived severally from distinct Algal types. The Psilotales he 

 regards as another entirely separate group, also of Algal origin, 

 but of geologically very late appearance. This highly polyphy- 

 letic hypothesis has something in common with the brilliant 

 speculations of Dr A. H. Church, whose essay on "Thalassio- 

 phyta and the Subaerial Transmigration" would have interested 

 Arber immensely, if he had lived to see it. 



