158 



NATURE 



[September 29, 192 1 



sperms (representing the early phase of the seed 

 plants), the ferns, the Sphenophylls, the Equi- 

 setales, and the Lycopods. These five lines were 

 probably all well differentiated in the Upper 

 Devonian flora. 



When we get back to the Middle and Lower 

 Devonian the case is completely altered. Not one 

 of the five phyla is here clearly represented, unless 

 it be the Spermophyta ; for these we have the 

 evidence of apparently g-ymnosperm-like stems. 

 Thus the field is left absolutely open to specula- 

 tion. We may imagine either that the various 

 phyla converged in some early vascular stock 

 (illustrated by the Psilophytales), or that they ran 

 back in parallel lines to independent origins among 

 the transmigrant Algae and, perhaps further still, 

 to separate races of purely marine plants. Both 

 views are represented in the publications of recent 

 authors. 



Dr. Arber, in his "Devonian Floras," main- 

 tained the early existence of three distinct lines 

 of descent : the Sphenopsida, Pteropsida, and 

 Lycopsida. Each of the three lines is described 

 as descended from thallophytic Algae of a distinct 

 type. Thus Arber's view was decidedly poly- 

 phyletic. 



Dr. Church, from quite a different point of view, 

 arrives at somewhat similar conclusions, but he 

 goes further. He says : " Speaking generally, it 

 appears safer to regard a ' race ' or ' phylum ' 

 as the expression of a group of organisms which 

 derived their special attributes from the equip- 

 ment of a preceding epoch, if not in one still 

 further back. Thus all the main lines of what 

 is now land flora must have been differentiated in 

 the Benthic epoch of the sea [i.e. as algal lines), 

 as all algal lines were differentiated in- the Plank- 

 ton phase. The possibility is not invalidated that 

 existing groups of land flora may trace back their 

 special line of progression to the flagellated life of 

 the sea, wholly independently of one another 

 (Pteridophyta).''' 



Thus the idea of independent parallel lines of 

 descent is carried to its extreme limit. " Each 

 phylum goes back the whole way, without any 

 connection with anything else." Of course, this 

 thorough-going polyphyletic conception is in- 

 volved in the doctrine already mentioned that 

 morphological differentiation was attained in the 

 sea before the transmigration. 



I have cited Dr. Arber and Dr. Church as 

 independent representatives, approaching the 

 question from quite different sides, of the poly- 

 phyletic or parallel-phyla hypothesis. The oppo- 

 site view, of convergent monophyletic races, 

 is also well supported. Prof. Halle, after 

 speaking of the possible relation of the Psilo- 

 phyton type to Lycopods on one hand and 

 ferns on the other, adds: "From this point of 

 view the whole pteridophytic stock would be 

 monophyletic, the Lycopsida and the Pteropsida 

 being derived from a common form already vascu- 

 lar. It would not thus be necessary to assume 

 a parallel evolution of a similar vascular system 

 along two different lines." 



NO. 2709, VOL. 108] 



Kidston and Lang, in the light of their Rhynie 

 discoveries, regard Halle's survey as "a fair 

 statement of the present bearing of the imperfectly 

 known facts." They add: "The geological age 

 and succession of the Early Devonian plants are, 

 on the whole, consistent with the origin of the 

 various groups of vascular Cryptogams from a 

 common source." We have already referred to 

 the bryophytic features, which have been recog- 

 nised in the Rhyniaceae. Kidston and Lang make 

 use of these to extend their tentative conclusions 

 to the Bryophyta. In concluding their third 

 memoir they say : "In Rhynia and Hornea we 

 have revealed to us a much simpler type of vascu- 

 lar Cryptogam than any with which we were 

 previously acquainted. This type suggests the 

 convergence of Pteridophyta and Bryophyta back- 

 wards to an algal stock. The knowledge of 

 Asteroxylon confirms and enriches our conception 

 of a more complex but archaic type of the vascu- 

 lar Cryptogams, which supports the idea of the 

 divergence of the great classes of Pteridophyta 

 from a common type, and links this on to the 

 simpler Rhyniaceae." The monophyletic view, 

 though stated with appropriate caution, could not 

 be more clearly expressed 



It is evidently impossible to decide between the 

 two theories in the present state of our know- 

 ledge ; we are now only beginning to acquire 

 some conception of the vegetation of Early 

 Devonian times. The discovery, however, of the 

 existence at that period of an unexpectedly simple 

 race of vascular plants to some extent favours a 

 monophyletic interpretation. To some minds, too, 

 the important points in which all existing Pteri- 

 dophyta, however diverse, agree will still suggest 

 a common origin not too remote. Among such 

 common characters may be mentioned the alterna- 

 tion of generations with the sporophyte predomi- 

 nant ; the development both of the spores and th« 

 sexual organs ; and the histology, especially of th< 

 vascular system and the stomata. The com- 

 munity of reproductive phenomena is explained 

 by Dr. Church on the principle that reproductive 

 phases are inevitable and are therefore the same 

 in all phyla. A like explanation may to a certain 

 extent be applicable to somatic features, some of 

 which may be the necessary consequences of the 

 sub-aerial transmigration. Thus a polyphyletic 

 hypothesis may no doubt be justified, but it \ 

 urgently needs to be supported by further evidence . 

 of the actual existence of separate stocks 

 among the earliest available records of a land 

 flora. 



The study of fossil botany has led to results of 

 the utmost Importance in widening our view ofi 

 the vegetable kingdom and helping to complete ; 

 the natural system, to use Solms-Laubach's old 

 phrase once more. One need mention only the 

 Mesozoic Cycadophytes, the Cordaltales, the 

 Pteridosperms, the Palaeozoic Lycopods and 

 Equlsetales, the Sphenophylls, and now, most 

 striking of all, the Psilophytales, to recall how 

 much has been gained. We have Indeed a wealth , 

 of accumulated facts, but from the point of view 



