The distinguished French palaeobotanists, Grand'Eury (C. Grand'Eury, "Sur les mutations de quelques Plantes fossiles du Terrain houiller". "Comptes Rendus", CXLII. page 25, 1906.) and Zeiller (R. Zeiller "Les Vegetaux fossiles et leurs Enchainements", "Revue du Mois", III. February, 1907.), are of opinion, to quote the words of the latter writer, that the facts of fossil Botany are in agreement with the sudden appearance of new forms, differing by marked characters from those that have given them birth; he adds that these results give more amplitude to this idea of Mutation, extending it to groups of a higher order, and even revealing the existence of discontinuous series between the successive terms of which we yet recognise bonds of filiation. (Loc. cit. page 23.)
If Zeiller's opinion should be confirmed, it would no doubt be a serious blow to the Darwinian theory. As Darwin said: "Under a scientific point of view, and as leading to further investigation, but little advantage is gained by believing that new forms are suddenly developed in an inexplicable manner from old and widely different forms, over the old belief in the creation of species from the dust of the earth." ("Origin of Species", page 424.)
It most however be pointed out, that such mutations as Zeiller, and to some extent Arber and Parkin, appear to have in view, bridging the gulf between different Orders and Classes, bear no relation to any mutations which have been actually observed, such as the comparatively small changes, of sub-specific value, described by De Vries in the type-case of Oenothera Lamarckiana. The results of palaeobotanical research have undoubtedly tended to fill up gaps in the Natural System of plants—that many such gaps still persist is not surprising; their presence may well serve as an incentive to further research but does not, as it seems to the writer, justify the assumption of changes in the past, wholly without analogy among living organisms.
As regards the succession of species, there are no greater authorities than Grand'Eury and Zeiller, and great weight must be attached to their opinion that the evidence from continuous deposits favours a somewhat sudden change from one specific form to another. At the same time it will be well to bear in mind that the subject of the "absence of numerous intermediate varieties in any single formation" was fully discussed by Darwin. ("Origin of Species", pages 275-282, and page 312.); the explanation which he gave may go a long way to account for the facts which recent writers have regarded as favouring the theory of saltatory mutation.
The rapid sketch given in the present essay can do no more than call attention to a few salient points, in which the palaeontological records of plants has an evident bearing on the Darwinian theory. At the present day the whole subject of palaeobotany is a study in evolution, and derives its chief inspiration from the ideas of Darwin and Wallace. In return it contributes something to the verification of their teaching; the recent progress of the subject, in spite of the immense difficulties which still remain, has added fresh force to Darwin's statement that "the great leading facts in palaeontology agree admirably with the theory of descent with modification through variation and natural selection." (Ibid. page 313.)
XIII. THE INFLUENCE OF ENVIRONMENT ON THE FORMS OF PLANTS. By Georg Klebs, PH.D.
Professor of Botany in the University of Heidelberg.
The dependence of plants on their environment became the object of scientific research when the phenomena of life were first investigated and physiology took its place as a special branch of science. This occurred in the course of the eighteenth century as the result of the pioneer work of Hales, Duhamel, Ingenhousz, Senebier and others. In the nineteenth century, particularly in the second half, physiology experienced an unprecedented development in that it began to concern itself with the experimental study of nutrition and growth, and with the phenomena associated with stimulus and movement; on the other hand, physiology neglected phenomena connected with the production of form, a department of knowledge which was the province of morphology, a purely descriptive science. It was in the middle of the last century that the growth of comparative morphology and the study of phases of development reached their highest point.
The forms of plants appeared to be the expression of their inscrutable inner nature; the stages passed through in the development of the individual were regarded as the outcome of purely internal and hidden laws. The feasibility of experimental inquiry seemed therefore remote. Meanwhile, the recognition of the great importance of such a causal morphology emerged from the researches of the physiologists of that time, more especially from those of Hofmeister (Hofmeister, "Allgemeine Morphologie", Leipzig, 1868, page 579.), and afterwards from the work of Sachs. (Sachs, "Stoff und Form der Pflanzenorgane", Vol. I. 1880; Vol. II. 1882. "Gesammelte Abhandlungen uber Pflanzen-Physiologie", II. Leipzig, 1893.) Hofmeister, in speaking of this line of inquiry, described it as "the most pressing and immediate aim of the investigator to discover to what extent external forces acting on the organism are of importance in determining its form." This advance was the outcome of the influence of that potent force in biology which was created by Darwin's "Origin of Species" (1859).