darwin and modern science-第57章
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ermophyta without pollen…tubes。 The pollen…tube method ultimately prevailed； becoming a constant 〃morphological character；〃 for no other reason than because； under the new conditions； it provided a more perfect mechanism for the accomplishment of the act of fertilisation。 We have still； in the Cycads and Ginkgo； the transitional case； where the tube remains short； serves mainly as an anchor and water…reservoir； but yet is able； by its slight growth； to give the spermatozoids a 〃lift〃 in the right direction。 In other Seed…plants the sperms are mere passengers； carried all the way by the pollen…tube； this fact has alone rendered the Angiospermous method of fertilisation through a stigma possible。
We may next take the seed itselfthe very type of a morphological character。 Our fossil record does not go far enough back to tell us the origin of the seed in the Cycadophyta and Pteridosperms （the main line of its development） but some interesting sidelights may be obtained from the Lycopod phylum。 In two Palaeozoic genera； as we have seen； seed…like organs are known to have been developed； resembling true seeds in the presence of an integument and of a single functional embryo…sac； as well as in some other points。 We will call these organs 〃seeds〃 for the sake of shortness。 In one genus （Lepidocarpon） the seeds were borne on a cone indistinguishable from that of the ordinary cryptogamic Lepidodendreae； the typical Lycopods of the period； while the seed itself retained much of the detailed structure of the sporangium of that family。 In the second genus； Miadesmia； the seed…bearing plant was herbaceous； and much like a recent Selaginella。 （See Margaret Benson； 〃Miadesmia membranacea； a new Palaeozoic Lycopod with a seed…like structure〃； 〃Phil。 Trans。 Royal Soc。 Vol。 199； B。 1908。） The seeds of the two genera are differently constructed； and evidently had an independent origin。 Here； then； we have seeds arising casually； as it were； at different points among plants which otherwise retain all the characters of their cryptogamic fellows； the seed is not yet a morphological character of importance。 To suppose that in these isolated cases the seed sprang into being in obedience to a Law of Advance （〃Vervollkommungsprincip〃）； from which other contemporary Lycopods were exempt； involves us in unnecessary mysticism。 On the other hand it is not difficult to see how these seeds may have arisen； as adaptive structures； under the influence of Natural Selection。 The seed…like structure afforded protection to the prothallus； and may have enabled the embryo to be launched on the world in greater security。 There was further； as we may suppose； a gain in certainty of fertilisation。 As the writer has pointed out elsewhere； the chances against the necessary association of the small male with the large female spores must have been enormously great when the cones were borne high up on tall trees。 The same difficulty may have existed in the case of the herbaceous Miadesmia； if； as Miss Benson conjectures； it was an epiphyte。 One way of solving the problem was for pollination to take place while the megaspore was still on the parent plant； and this is just what the formation of an ovule or seed was likely to secure。
The seeds of the Pteridosperms； unlike those of the Lycopod stock； have not yet been found in statu nascendiin all known cases they were already highly developed organs and far removed from the cryptogamic sporangium。 But in two respects we find that these seeds； or some of them； had not yet realised their possibilities。 In the seed of Lyginodendron and other cases the micropyle； or orifice of the integument； was not the passage through which the pollen entered； the open neck of the pollen…chamber protruded through the micropyle and itself received the pollen。 We have met with an analogous case； at a more advanced stage of evolution； in the Bennettiteae； where the wall of the gynaecium； though otherwise closed； did not provide a stigma to catch the pollen； but allowed the micropyles of the ovules to protrude and receive the pollen in the old gymnospermous fashion。 The integument in the one case and the pistil in the other had not yet assumed all the functions to which the organ ultimately became adapted。 Again； no Palaeozoic seed has yet been found to contain an embryo； though the preservation is often good enough for it to have been recognised if present。 It is probable that the nursing of the embryo had not yet come to be one of the functions of the seed； and that the whole embryonic development was relegated to the germination stage。
In these two points； the reception of the pollen by the micropyle and the nursing of the embryo； it appears that many Palaeozoic seeds were imperfect； as compared with the typical seeds of later times。 As evolution went on； one function was superadded on another； and it appears impossible to resist the conclusion that the whole differentiation of the seed was a process of adaptation； and consequently governed by Natural Selection； just as much as the specialisation of the rostellum in an Orchid； or of the pappus in a Composite。
Did space allow； other examples might be added。 We may venture to maintain that the glimpses which the fossil record allows us into early stages in the evolution of organs now of high systematic importance； by no means justify the belief in any essential distinction between morphological and adaptive characters。
Another point； closely connected with Darwin's theory； on which the fossil history of plants has been supposed to have some bearing； is the question of Mutation； as opposed to indefinite variation。 Arber and Parkin； in their interesting memoir on the Origin of Angiosperms； have suggested calling in Mutation to explain the apparently sudden transition from the cycadean to the angiospermous type of foliage； in late Mesozoic times； though they express themselves with much caution； and point out 〃a distinct danger that Mutation may become the last resort of the phylogenetically destitute〃！
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 plantsthat 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
