156 



NATURE 



[September 29, 192 1 



the rooted sea-weeds succeeding- the free- 

 swimming plankton, and then the continents 

 slowly emerging and the drama of the trans- 

 migration, as the plants of the rock-pools and 

 shallows fit themselves step by step for sub-aerial 

 life when the dry land appears. It is a striking 

 picture that is thus displayed to our view — 

 whether in all respects a faithful one is another 

 question ; we must not expect impossibilities. 

 The doubts which have been raised relate first to 

 the assumed world-wide ocean, which seems not 

 to be generally accepted by geologists. If con- 

 tinental ridges existed from the first {i.e. from 

 the original condensation of watery vapour to 

 form seas), the colonisation of the land may have 

 followed other lines and have Jiappened repeatedly. 

 Perhaps, after all, that would not greatly affect the 

 botanical aspects of the transmigration. 



Dr. Church believes that the chief morphologi- 

 cal characters of the land flora were first out- 

 lined in the sea ; that such characters were not 

 newly assumed after transmigration, but that they 

 merely represent an adaptation to sub-aerial con- 

 ditions of a differentiation already attained at the 

 phase of marine phytobenthon (rooted sea-weeds). 

 At the same time it is not suggested that any 

 existing class of sea-weeds can be taken as repre- 

 senting the ancestry of the land flora ; the trans- 

 migrant races are, as Algae, extinct — they may 

 have been Green Algse of a high grade of organ- 

 isation, on a level now perhaps most nearly repre- 

 sented by the highest of the Brown Sea-weeds. 



Thus the transmigrants, which were destined 

 to become the. parents of the land flora, are pic- 

 tured as already highly organised and well dif- 

 ferentiated plants, which only needed to provide 

 themselves with absorptive instead of merely 

 anchoring roots, and with a water-conducting 

 system (xylem and stomata) in order to fit them- 

 selves for sub-aerial life, while, on the reproduc- 

 tive side, the great change remaining to be accom- 

 plished was the adaptation of the spores to trans- 

 port by air instead of by water. 



Some botanists find a difficulty in accepting the 

 suggestion that plants already elaborately fitted 

 out for a marine life could have survived the 

 transition, however gradual, to a totally different 

 environment. Such thinkers prefer to believe that 

 lower forms may have been more adaptable, and 

 that morphological differentiation had, in a great 

 degree, to start afresh when the land was first 

 invaded. My own sympathies, I may say, are 

 here with Dr. Church, for I have long inclined to 

 the belief that the vascular plants were, in all 

 probability, derived from the higher Thallo- 

 phytes. The view of the late Prof. Lignier, 

 now so widely accepted, that the leaf, at least in 

 the megaphyllous or fern-like vascular plants, was 

 derived from specialised branch-systems of a 

 thallus, assumes, at any rate, that the immediate 

 ancestors possessed a well-developed thallus, such 

 as is now known only among the higher Algae. 



The question now arises, how far have we any 

 •evidence from the rocks which may bear on the 1 

 NO. 2709, VOL. 108] 



transmigration and on the nature of the early 

 land flora? Quite recent discoveries, especially 

 those from the famous Rhynie Chert-bed, have 

 shown that in Early Devonian times certain 

 remarkably simple land-plants existed, which in 

 general configuration were no more advanced than 

 some very ordinary sea-weeds of the present day. 

 At the same time these plants were obviously 

 fitted for terrestrial life, as shown by the presence 

 of a water-conducting tissue and stomata, and by 

 the manifestly air-borne spores. These simplest 

 land-plants are the Rhyniaceae (Rhynia and 

 Hornea), while the third genus, Asteroxylon, was 

 more advanced and further removed from any 

 possible transmigrant type. 



Dr. Arber was so impressed by the primitive 

 character of Rhynia (the only one of these genera 

 then known) that he boldly called it a Thallophyte, 

 while recognising, in respect of anatomical struc- 

 ture, an intermediate position on the way to 

 Pteridophyta. This is not really very different 

 from the view taken by the investigators them- 

 selves, though they call the plants Pteridophytes, 

 which they certainly are if we go by internal 

 structure rather than external morphology. But 

 if, as Kidston and Lang suggest, the Rhyniaceae 

 " find 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," they,] 

 must have been very primitive indeed, and might 

 even be regarded as fairly representing the true 

 transmigrants w'hich had not long taken to the 

 land. 



It is true that the Middle Devonian is much too 

 late a period for the original transmigration (I 

 believe there is some evidence for land-animals in., 

 the Lower Silurian), but one may argue that some 

 of the transmigrant forms may have survived as 

 late as the Devonian, just as the Selaginella type 

 seems to have gone on with little change from 

 the carboniferous to the present time. There 

 must have been many such survivals of earlier 

 forms in the Devonian period, if Arber was right 

 in regarding all the characteristic plants of the 

 Psilophyton flora as ' ' much more probably 

 Thallophyta than Pteridophyta." There is, in 

 fact, no doubt that the earlier Devonian flora is 

 turning out to have been on the whole more 

 peculiar and more unlike the higher plants than 

 we realised a few years ago. The Early Devonian 

 plants cannot usually be referred ' to any of the 

 recognised groups of Pteridophytes, and this is 

 not owing to our imperfect knowledge, for it is 

 just in those cases where the plants are most 

 thoroughly known that their unique systematic 

 position is most manifest. Arber cafled all the 

 plants in question " Procormophyta " — an appro- 

 priate name. As Kidston and Lang point out in 

 their later work, the three groups — Pteridophyta, 

 Bryophyta, and Algae — are brought nearer 

 together by the Rhynie fossils. 



Yet there is evidence that about the same period 

 stems with the -highly organised structure of 

 Gvmnospermous trees already existed. I refer to 



