LIXNEAN SOCIETY OF LONDON. 67 



alike, aud homologous one with the otlier in all their parts, iu cases 

 where they are exposed to like conditions. 



Tims the idea has once more gained ground that the vascular 

 plant — the sporophvte — like the gametophyte or prothallus, may 

 itself be a modified thallus : iu this way the whole field of 

 comparison between the higher plants and the Thallopliyta is 

 once more opened up. This conception adds enormously to the 

 interest of the older types of v^ascular plants, for there is now 

 always the possibility that among them we may succeed in tracing 

 their organs — leaf, stem, and root — a step or two nearer to their 

 origin. There was never the remotest chance of this so long as 

 the plant was supposed to be derived from a sporogonium, for it 

 was obvious that the farther back we went in geological history, 

 the less like a sporogonium did plants prove to be. 



As we shall presently see, the new views of alternation, 

 involving the tlialloid origin of the vascular plant, have already 

 proved fertile in evolutionary ideas ; the palaeontologist, however, 

 will do well to maintain a cautious position with regard to the 

 application of these conceptions to fossil plants. Though we may 

 now have a tenable theory of the origin of vascular plants, and 

 it is theoretically possible that we may be able to trace some of 

 the stages iu their evolution from thalloid ancestors, it is yet 

 extremely doubtful whether the fossil record goes far enough back 

 to help us appreciably in such an attempt. It cannot be too 

 strongly emphasized that the earliest known land-plants were 

 already highly advanced and varied types, very far removed 

 from any thalloid ancestr}^. It is possible that here and there 

 a primitive character may have lingered, but the presumption is 

 always against it. 



I shall therefore only touch on theories of the derivation of 

 vascular plants in so far as they affect our views of the inter- 

 relations of their main groups, for it is on questions of the latter 

 kind that new light has been thrown by the investigations, largely 

 palseobotanical, of the last few years. 



I may here refer to a remark of Dr. Gaskell's, iu his opening 

 «peech in the discussion on the Origin of Vertebrates, to which 

 we listened with such deep interest a month or two back. 

 Dr. Graskell used these words : — " It seems to me highly probable 

 that this same law of upward progress, viz., that each successive 

 group has arisen from some member of the highest group existing 

 at the time, holds good also for the Vegetable Kingdom, especially 

 in view of the statement recently made that Phanerogaius arose 

 from Cycads. I hope that the President may see his way to offer 

 a few remarks on this aspect of the question " *. 



There was no time then to deal with Dr. Gaskell's point, and I 

 promised to refer to it at the Anniversary Meeting. The question 



* Discussion on the Origin of Verteljrates, Proc. Linn, Soc, Session 122, 

 1909-10, p. 12. 



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