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THE BOTANICAL WORK OF DARWIN.

CHARLES DARWIN's botanical work falls naturally into two periods, of which the first ends with the publication of the Origin of Species in 1859, while the second begins with the appearance of the Fertilization of Orchids in 1862.

The chronological grouping, however, only holds good to a limited extent, and the work can be more accurately classified according to its nature and aim. Thus the botanical part of the Variation of Animals and Plants is principally characteristic work of the first period, though the book appeared six years after the Fertilization of Orchids; but here, as in many other instances, the date of publication is no guide as to the date of research, and, as a fact, the work in question was largely the fruit of earlier years.

In the first or evolutionary period, plants, like animals, were the material on which he tested his 'species theory.' In the second period he worked in detail at varied problems in plant physiology.

I believe the classification of his work into evolutionary and physiological will be a convenient basis for discussing his results, although a somewhat vague line separates the classes. Thus, his original interest in the fertilization of flowers was purely evolutionary. 'I was led,' he says, 'to attend to the cross-fertilization of flowers by the aid of insects, from having come to the conclusion, in my speculations on the origin of species, that crossing played an important part in keeping specific forms constant¹.' But he continued to study the means of fertilization of Orchids, &c.,

¹ Life and Letters, Vol. i, p. 90.

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principally because of his irresistible desire to understand the machinery of living things. It is true that in elucidating the machinery he supplied the most brilliant evidence in favour of the validity of natural selection as the great moulding force in Nature. But I do not think this was his object, it was rather a by-product of work carried on for the love of doing it. It is true that he felt the importance of the evidence in regard to evolution, for he says:—'It will perhaps serve to show how natural history may be worked under the belief of the modification of species¹.'

During the long years of the first period he was learning to know plants as he used them in the building of his theory; and then the tables were turned, and the theory served him as a powerful engine to break still further into the secrets of plants, and the engine seemed all the more marvellously effective because he employed it on problems already half conquered in his earlier work. Instances of the pervasion of the second group of works by evolutionary characteristics will be given later.

Evolutionary Period.

I shall not attempt to deal at any length with the Botany of the evolutionary period; the nature of the work may be fairly estimated by a study of the Origin, first edition, 1859; Animals and Plants, 1868; and the Life and Letters. Taking the Origin first, a striking feature is the use made of a mass of botanical facts in parts of the science in which he had no special first-hand knowledge. Mr. Huxley has pointed out that in Zoology, by the eight years given to the Cirripedes, he had made himself a master in the trade; he knew the raw material, and could judge of the theoretical strain which that material would bear. But in Botany he had no such training. He had to test his theories on difficult problems in vegetable morphology, classification, and distribution, with an outfit of knowledge based almost entirely on reading, and on what he had learned from Henslow at Cambridge.

¹ Life and Letters, Vol. iii, p. 254.

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It must, however, be remembered that his Cirripede work had done much more than make him a zoologist; it made him, in a general sense, a systematist and this must have been of incalculable value.

In the matter of geographical distribution he had at least enjoyed the great advantage of seeing the vegetation of the world, but he had seen it with ignorant eyes. In 1836, immediately after his return from his voyage, he wrote:— 'I felt very foolish when Mr. Don remarked on the beautiful appearance of some plant with an astounding long name, and asked me about its habitation. Some one else seemed quite surprised that I knew nothing about a Carex from I do not know where. I was at last forced to plead most entire innocence, and that I knew no more about the plants which I had collected than the man in the moon¹.'

To the end of his life he never made any pretence to be a botanist, orat best 'one of those botanists who do not know one plant from, another,' a saying, attributed to Nägeli, which he was fond of quoting. Thus, too, he wrote to Asa Gray on being elected to the Botanical Section of the French Institute:—'It is rather a good joke that I should be elected in the Botanical Section, as the extent of my knowledge is little more than that a daisy is a compositous plant, and a pea a leguminous one².' He was in fact guilty of evolution, but with extenuating botanical circumstances.

It is perhaps not out of place to call attention, as I have done, to the poverty of professional training with which he attacked evolution on its botanical side; it not merely brings out his power of using the work of others, but it also brings out the value of his point of view, that he should, equipped as he was, have revolutionized botanical geography. Again, it makes clear to us the supreme value to him and to science of his lifelong friendship with Sir J. D. Hooker. All readers of the Life and Letters must have been struck with the paramount importance of the Hooker correspondence as

¹ Life and Letters, Vol. i, p. 275.

² Ibid., Vol. iii, p. 224.

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a record of Darwin's life. But to me the thought is more pressingly present, that without Hooker's constant help his great task would not have been carried out on the botanical side. Only part of these letters are given in the Life and Letters, when the remainder are published they will only heighten the impression of the value to science of this memorable friendship. It was not merely information, guidance, explanation, that he received, but an inspiriting companionship, a fresh and vigorous influence giving continuous refreshment to the solitary worker.

The following list of subjects, taken at random from my father's letters to Sir Joseph, give an idea of the subjects discussed during the evolutionary period:—The dispersal of seeds, continental extension, geographical barriers, the arctic Flora, alpine plants, large genera varying, coal plants, island Floras¹, aberrant genera, direct action of conditions, rarity and extinction, sterility, graft hybrids. Many of these were also the subject of correspondence with Asa Gray, H. C. Watson, and others, with results familiar to us in the Origin.

One of the few pieces of his published botanical researches made use of in the Origin was his series of experiments on the flotation and vitality of seeds in salt water. The chief mass of his work on crossing plants was later than 1859, and it will be found that in the Origin his material comes chiefly from Knight, Kölreuter, and Gärtner.

He used to sneer at himself as a compiler:—'The inaccuracy of the blessed gang (of which I am one) of compilers passes all bounds¹.' But 'compiling' in his sense was not an easy art, the first requisite being an instinctive power of knowing the trustworthy authorities from the untrustworthy. In writing to Professor Huxley this was italicized, The difficulty is to know what to trust. He adds too, 'I have picked up most by reading really numberless special treatises, and all agricultural and horticultural journals; but it is a work of long years. ... I have found it very important associating with fanciers and breeders².'

¹ Life and Letters, Vol. ii, p. 281, note.

² Ibid., p. 281.

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The mass of information gathered in the last-named way and by reading is given in his Variation of Animals and Plants under Domestication; which, like the Descent of Man and the Expression of the Emotions, were amplifications of parts of the Origin, in which he gave in full the evidence referred to in that so-called 'Abstract.' In the Variation of Animals and Plants he was also able to include more of his own researches, partly because, as already explained, it was published in 1868, six years after the Orchid book, and partly because it was a more appropriate place for minute observations. Here, for instance, are given the remarkable facts on the similarity, or rather identity, of the seeds of various kinds of Brassica, in which the leaves or other vegetative parts have by man's selective power come to differ widely¹.

Among the subjects especially interesting to the botanist as distinguished from the horticulturist may be mentioned the contents of the well-known chapter xi of Vol. I, 'on bud variation, and on certain anomalous modes of reproduction and variation.' In Vol. II, again, there are well-known chapters or paragraphs on crossing, on Knight's Law², on the sterility of cultivated varieties, on the good effect of crossing and on self-sterility, where his own work and his correspondence with Fritz Mtiller are prominent; on the stimulating effect of changed conditions, and on the complex problem of sterility due to changed conditions; on the difference in fertility between varieties and species when crossed, and on hybridism; on the direct action of conditions.

My father's correspondence with Fritz Müller was, in its bearing on his work, second in importance only to that with Hooker. He had for Müller a stronger personal regard than that which bound him to his other unseen friends. Müller 's letters were vividly interesting, with their constant stream of new observation on many biological subjects. Moreover,

¹ Animals and Plants, ed. 1, Vol. i, p. 323. The collections of seeds made for this purpose are now in the Botanical Museum at Cambridge.

² I have elsewhere discussed Knight's Law in relation to Darwin's work; see Nature, 1898.

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there was, by an unformulated arrangement, a certain community of research on many subjects. For instance, on orchid-fertilization, self-sterility, heterostylism, and climbing plants the facts supplied by Müller were important contributions to the building up and extending of Darwin's theories.

It is needless to say more on the Variation of Animals and Plants, the book is in the hands of every one, and is familiar to botanists. I regret that space forbids me to quote the true and forcible description of the effect of the book on botanists and horticulturists, given by Sir W. Thiselton-Dyer in his classical essay on the Botanical Work of Darwin¹.

Second or Physiological Period.

The work of this period, though distinguishable in a certain sense from the purely evolutionary work, is yet bound to it by many interwoven ties. Thus, the Orchid book (taken in conjunction with Cross and Self-Fertilization) is the amplification of a passage in the first edition of the Origin². 'The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason to believe (as will hereafter be more fully alluded to), would produce very vigorous seedlings, which consequently would have the best chance of flourishing and surviving.'

Its connexion, from another point of view, with the evolutionary work has already been made clear, and I have indicated its value as a demonstration of the efficiency of his theory of evolution as a guide in natural history work. He acknowledged the value of his Orchid work in relation to natural selection:—'I can show the meaning of some of the apparently meaningless ridges [and] horns; who will now venture to say that this or that structure is useless³?'

It is not every man who can prove so convincingly the

¹ Charles Darwin, Nature Series, 1882, p. 38.

² p. 92.

³ Life and Letters, Vol. iii, p. 254.

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temper of the tool he has forged, or who can hope to see his single arm multiply a thousandfold, and a great guild of craftsmen working with his aims and his methods. Yet this is what has happened, for the great mass of biological or natural history work of the last thirty years is part of that harvest of which the Fertilization of Orchids was the first-fruits. What Mr. Huxley has said is true here:—'Even a cursory glance at the history of the biological sciences during the last quarter of a century is sufficient to justify the assertion, that the most potent instrument for the extension of the realm of natural knowledge which has come into men's hands since the publication of Newton's Principia is Darwin's Origin of Species¹.'

But it is not possible to judge the Orchid book until Darwin's researches on Cross and Self Fertilization, published in 1876, are taken into account.

In the Orchid book he showed that certain machinery exists for insuring cross-fertilization. In Cross and Self Fertilization he showed, for the first time, the definitely measurable effect of cross-fertilization, and thus expressed in figures the value of the selective force at work in modifying the floral mechanism. And in Cross and Self Fertilization he generalized the position one degree further; by showing that the value of crossing does not depend on the union of two individuals as representatives of different sexes, but as representatives of different conditions of life. The light in this way thrown on the meaning of sexual reproduction is, in my judgement, one of his greatest achievements in science.

A great deal of work, part only of which has been published, was done on the floral mechanism of the Papilionaceae, on Leshenaultia, on Melastomaceae, and on Clarkia. In the two last-named examples it was the possession of two different sets of anthers which interested him, a state of things which has been shown in some cases to be a case of division of labour, one set supplying pollen as an attraction, while the remainder serve as fertilizers. The Melastomaceae are a good

¹ Life and Letters,Vol. ii, p. 204.

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example of his persistence; they began to interest him in about 1862, and he was still hoping to attack them in 1881, as illustrated in letters to Sir W. Thiselton-Dyer and others, which I hope soon to publish.

His next great work in the domain of fertilization was his papers on Heterostylism, the well-known researches on Primula, Linum, Lythrum, &c., which were afterwards worked up with other cognate matter into his book on Forms of Flowers, 1877. This work was extremely laborious, but with the labour went an especial delight in reading the riddle. Thus he wrote :— 'I do not think anything in my scientific life has given me so much satisfaction as making out the meaning of the structure of these plants¹.' While the delight of unravelling a biological puzzle was, as usual with him, the chief incentive to work, he did not overlook any fraction of the lessons to be learned from his results. In the case of heterostylism, he thought that the essential value of the problem lay in its bearing on hybridism. The parallelism between hybridization and certain heterostyled unions is wonderfully close, so that it is hardly an exaggeration to say that 'illegitimately' reared seedlings are hybrids between members of a single species. This fact gave a death-blow to the doctrine (which has been so long in dying) that differences in sexual constitution are 'the very touchstone of specific distinction².'

Another group of his researches was connected with the irritability of plants as exhibited in movement. The earliest of these was his work on Climbing Plants, 1865, in which he followed Mohl and Palm, but with the addition of so much new matter that he practically made the subject his own.

In his Insectivorous Plants, 1875, he also discovered and thoroughly investigated a number of remarkable instances of plant movement. His account of the sensitiveness of Drosera to excessively minute weights, though since shown by Pfeffer to bear a somewhat different interpretation, yet remains one of the most wonderful instances of vegetable irritability.

¹ Life and Letters, Vol. i, p. 91.

² Forms of Flowers, p. 276.

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His views on aggregation have been shown to be incorrect, but remain as the starting-point of a curious subject. The work on insectivorous plants is, however, more remarkable for the boldness and originality of the central idea of the book than for anything else. As he has said, 'The fact that a plant should secrete, when properly excited, a fluid containing an acid and ferment, closely analogous to the digestive fluid of an animal, was certainly a remarkable discovery¹.'

In his book on Climbing Plants he wrote:—'The conclusion is forced on our minds that the capacity of revolving, on which most climbers depend, is inherent, though undeveloped, in almost every plant in the vegetable kingdom².' This, extended and modified, forms the central conception of his book on the Power of Movement in Plants, 1880. He showed that circurnnutation is a widely spread phenomenon, that practically all plants carry on their growth in a rhythmic manner, which is identical on a small scale with the revolving nutation of climbing plants. But, as is well known, he went much farther than this, and attempted to prove that the movements of plants have been evolved by various modifications of circumnutation. It is a point of view which harmonizes admirably with his evolutionary views, namely, the conception of a plant attaining an adaptive movement (e. g. geotropism) by selection from a tentative series of movements—in a way that calls to mind the selection and summation in a given direction of morphological variation. This view has not been accepted by botanists, and personally I doubt whether it should be accepted in the form in which it is stated; for I doubt whether we know enough of the machinery of curvature, as distinct from the machinery of rectilinear growth, to understand the connexion between the two. But, as Sir William Thiselton-Dyer has said, 'No one can doubt the importance of what Mr. Darwin has done in showing that for the future the phenomena of plant movement can and indeed must be studied from a single point of view³.'

¹ Life and Letters, Vol. i, p. 96.

² Climbing Plants, p. 205.

³ Charles Darwin, Nature Series, 1882, p. 41.

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In my opinion the most striking feature of the book is its rehabilitation of Dutrochet's¹ theory, that vegetable movements are 'spontanes, executes a l'occasion de l'influence d'un agent exterieur et non des mouvements imprimes par cet agent.' This remains a signal service to plant physiology, whether or no we accept Darwin's views as to the role of circumnutation.

The localization of geotropic and heliotropic sensitiveness in roots and in certain seedlings is perhaps the most striking of the discoveries published in the Power of Movement in Plants. By some persons these statements and conclusions were received with incredulity or contempt. The proof of their substantial accuracy is due to Pfeffer² and his pupils, Rothert, and Czapek.

Towards the end of his life my father more than once spoke of his physiological researches as being undertaken in place of the more trying evolutionary work, for which he felt himself too old. This has always struck me as remarkable; I should have believed that an old man in bad health would have more easily returned to the work on which he had spent his best years; I should have supposed it more difficult to attack a comparatively new subject with new methods and new lines of thought. When I remember the amount of labour necessitated by the Power of Movement, I am astonished at his courage and unflagging energy. His manner of attack has been so truly described by Sir W. Thiselton-Dyer, that I cannot resist the pleasure of quoting it³. 'He turned his attention to plants doubtless because they were convenient objects for studying organic phenomena in their least complicated forms; and this point of view, which, if one may use the expression without disrespect, had something of the

¹ Pfeffer was the first to call attention to Dutrochet's remarkable utterance. In my address to Section D of the British Association, 1891, I have expressed my appreciation of the importance of Pfeffer's contributions to the study of the irritability of plants.

² The most brilliant demonstration of the 'brain-function' of the root-tip was published by Pfeffer in the Annals of Botany, 1894.

³ Charles Darwin, Nature Series, p. 43.

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amateur about it, was in itself of the greatest importance. For, from not being, till he took up any point, familiar with the literature bearing on it, his mind was absolutely free from any prepossession. He was never afraid of his facts or of framing any hypothesis, however startling, which seemed to explain them. ... In any one else such an attitude would have produced much work that was crude and rash. But Mr. Darwin—if one may venture on language which will strike no one who had conversed with him as overstrained—seemed by gentle persuasion to have penetrated that reserve of nature which baffles smaller men.'

It may have been an unconscious perception of this quality which forced one, while recognizing the boldness of his course, to feel certain of his success.

The portrait of Charles Darwin is by Messrs. Maull and Fox, who have been good enough to permit its reproduction. The date of the photograph is probably 1854; it is, however, impossible to be certain on this point, the books of Messrs. Maull and Fox having been destroyed by fire. The reproduction is by Mr. Dew-Smith, who has been at some disadvantage, having only an old and faded print to work from.

FRANCIS DARWIN.

September, 1899.