II. PLANTS.
By D.H. SCOTT, F.R.S.
President of the Linnean Society.
There are several points of view from which the subject of the present essay may be regarded. We may consider the fossil record of plants in its bearing: I. on the truth of the doctrine of Evolution; II. on Phylogeny, or the course of Evolution; III. on the theory of Natural Selection. The remarks which follow, illustrating certain aspects only of an extensive subject, may conveniently be grouped under these three headings.
I. THE TRUTH OF EVOLUTION.
When "The Origin of Species" was written, it was necessary to show that the Geological Record was favourable to, or at least consistent with, the Theory of Descent. The point is argued, closely and fully, in Chapter X.
"On the Imperfection of the Geological Record," and Chapter XI. "On the Geological Succession of Organic Beings"; there is, however, little about plants in these chapters. At the present time the truth of Evolution is no longer seriously disputed, though there are writers, like Reinke, who insist, and rightly so, that the doctrine is still only a belief, rather than an established fact of science. (J. Reinke, "Kritische Abstammungslehre", "Wiesner-Festschrift", page 11, Vienna, 1908.)Evidently, then, however little the Theory of Descent may be questioned in our own day, it is desirable to assure ourselves how the case stands, and in particular how far the evidence from fossil plants has grown stronger with time.
As regards direct evidence for the derivation of one species from another, there has probably been little advance since Darwin wrote, at least so we must infer from the emphasis laid on the discontinuity of successive fossil species by great systematic authorities like Grand'Eury and Zeiller in their most recent writings. We must either adopt the mutationist views of those authors (referred to in the last section of this essay) or must still rely on Darwin's explanation of the absence of numerous intermediate varieties. The attempts which have been made to trace, in the Tertiary rocks, the evolution of recent species, cannot, owing to the imperfect character of the evidence, be regarded as wholly satisfactory.
When we come to groups of a somewhat higher order we have an interesting history of the evolution of a recent family in the work, not yet completed, of Kidston and Gwynne-Vaughan on the fossil Osmundaceae. ("Trans. Royal Soc. Edinburgh", Vol. 45, Part III. 1907, Vol. 46, Part II. 1908, Vol. 46, Part III. 1909.) The authors are able, mainly on anatomical evidence, to trace back this now limited group of Ferns, through the Tertiary and Mesozoic to the Permian, and to show, with great probability, how their structure has been derived from that of early Palaeozoic types.
The history of the Ginkgoaceae, now represented only by the isolated maidenhair tree, scarcely known in a wild state, offers another striking example of a family which can be traced with certainty to the older Mesozoic and perhaps further back still. (See Seward and Gowan, "The Maidenhair Tree (Gingko biloba)", "Annals of Botany", Vol. XIV. 1900, page 109; also A. Sprecher "Le Ginkgo biloba", L., Geneva, 1907.)On the wider question of the derivation of the great groups of plants, a very considerable advance has been made, and, so far as the higher plants are concerned, we are now able to form a far better conception than before of the probable course of evolution. This is a matter of phylogeny, and the facts will be considered under that head; our immediate point is that the new knowledge of the relations between the classes of plants in question materially strengthens the case for the theory of descent. The discoveries of the last few years throw light especially on the relation of the Angiosperms to the Gymnosperms, on that of the Seed-plants generally to the Ferns, and on the interrelations between the various classes of the higher Cryptogams.
That the fossil record has not done still more for Evolution is due to the fact that it begins too late--a point on which Darwin laid stress ("Origin of Species" (6th edition), page 286.) and which has more recently been elaborated by Poulton. ("Essays on Evolution", pages 46 et seq., Oxford, 1908.) An immense proportion of the whole evolutionary history lies behind the lowest fossiliferous rocks, and the case is worse for plants than for animals, as the record for the former begins, for all practical purposes, much higher up in the rocks.
It may be well here to call attention to a question, often overlooked, which has lately been revived by Reinke. (Reinke, loc. cit. page 13.) As all admit, we know nothing of the origin of life; consequently, for all we can tell, it is as probable that life began, on this planet, with many living things, as with one. If the first organic beings were many, they may have been heterogeneous, or at least exposed to different conditions, from their origin; in either case there would have been a number of distinct series from the beginning, and if so we should not be justified in assuming that all organisms are related to one another. There may conceivably be several of the original lines of descent still surviving, or represented among extinct forms--to reverse the remark of a distinguished botanist, there may be several Vegetable Kingdoms! However improbable this may sound, the possibility is one to be borne in mind.