Augustin Pyrame de Candolle[37] (1778-1841) belongs to the number of those distinguished investigators of nature, who at the end of the last and the beginning of our own century made their native city Geneva a brilliant centre of natural science. De Candolle was the contemporary and fellow-countryman of Vaucher, Theodore de Saussure, and Senebier. Physics and physiology especially were being successfully cultivated at that time in Geneva, and Pyrame de Candolle was attracted to these studies; among his youthful efforts are some important investigations into the effect of light on vegetation, and the contributions which he made to vegetable physiology in his great work on that subject will be noticed in a later portion of this history. De Candolle turned his attention to all parts of theoretical and applied botany, but his importance for the history of the science lies chiefly in the direction of morphology and systematic botany, and it is this which we will now proceed to describe.
The amount and compass of De Candolle’s labours as a systematic and descriptive botanist exceed those of any writer before or after him. He wrote a series of comprehensive monographs of large families of plants, and published a new edition of De Lamarck’s large ‘Flore Française’ substantially altered and enlarged; and in addition to these and many similar works and treatises on the geographical distribution of plants, he set on foot the grandest work of descriptive botany that is as yet in existence, the ‘Prodromus Systematis Naturalis,’ in which all known plants were to be arranged according to his natural system and described at length,—a work not yet fully completed, and in which many other descriptive botanists of the last century participated, but none to so large an extent as De Candolle, who alone completed more than a hundred families. It is not possible to give an account in few words of the service rendered to botany by such labours as these; they form the real empirical basis of general botany, and the better and more carefully this is laid, the greater the security obtained for the foundations of the whole science.
But a still higher merit perhaps can be claimed for De Candolle, inasmuch as he not only like Jussieu elaborated the system and its fundamental principles in his descriptive works, but developed the theory, the laws of natural classification, with a clearness and depth such as no one before him had displayed. To this purpose he applied morphological researches, which in profundity and wealth of thought and in the fruitfulness of their results for the whole domain of systematic botany far surpassed all that Linnaeus and Jussieu had accomplished, and show us that while engaged in his splendid labours in descriptive botany he had caught during his ten years’ residence in Paris the true spirit of modern investigation of nature, as it had been developed by the French naturalists of the end of the previous century. Scarcely a trace is to be found in De Candolle of the scholasticism of Cesalpino and Linnaeus, which occasionally makes its appearance even in Jussieu. For instance, he dealt with morphology as essentially the doctrine of the symmetry of form in plants, that is, he found the basis of morphological examination in the relative position and numbers of the organs, disregarding their physico-physiological properties as of no account from the morphological point of view. He was therefore the first who recognised the remarkable discordance between the morphological characters of organs, which are of value for systematic purposes, and their physiological adaptations to the conditions of life, though it must at the same time be acknowledged, that he did not consistently carry out this principle, but committed grave offences against it in laying down his own system. It is a point of the highest interest in De Candolle’s morphological speculations, that he was the first who endeavoured to refer certain relations of number and form to definite causes, and thus to distinguish what is primary and important in the symmetry of plants from merely secondary variations, as is seen in his doctrine of the abortion and adherence of organs. In these distinctions De Candolle laid the foundation of morphological views, which, though now modified to some extent, do still contain the chief elements of morphology and the natural system; but his morphological speculations were confined to the domain of the Phanerogams, and chiefly advanced the theory of the flower; a morphology of the Cryptogams was as little to be thought of in the condition of microscopy before 1820, as the application of the history of development to the establishment of morphological theories.
De Candolle published his morphology or doctrine of symmetry and his theory of classification together in a book which appeared first in 1813, with the title, ‘Théorie Élémentaire de la botanique ou exposition des principes de la classification naturelle et de l’art de décrire et d’étudier les végétaux,’ and again in 1819 in an improved and enlarged edition. The second edition will be the one referred to in the further account of his views. The second chapter of the second book concerns us most at present. After alluding to the fact, that anatomy and physiology are concerned with the structure of the individual organ only so far as the power to fulfil its proper function depends on the structure, he points out that the physiological point of view is no longer sufficient when we are engaged in comparing the organs of different plants. Though it is true that the function of the organs is the most important for the life and permanence of the individual, yet we find these functions modified in the case of homologous organs in different plants; for the natural classification we must take into consideration only the entire system of organisation, that is, the symmetry of the organs. All organisms of a kingdom, he continues, have the same functions with slight modifications; the immense amount of variation in systematically different species depends therefore only on the way in which the general symmetry of structure varies. This symmetry of the parts, the discovery of which is the great object in the investigation of nature, is nothing more than the sum total (l’ensemble) of the positional relations of the parts. Whenever these relations (disposition) are regulated according to the same plan, the organisms exhibit a certain general resemblance to one another, independently of the form of the organs in detail; when this general resemblance is perceived, without any attempt to give any account of it in the detail, we have what has been called habitual relationship; but it is the task of the doctrine of symmetry to resolve this likeness of habit into its elements, and to explain its causes. Without this study of symmetry it may easily happen that two different kinds of symmetry may be supposed to be alike, because they seem outwardly alike to our senses, just as forms of crystals of different systems may be confounded together for want of careful examination; the chief thing is to know the plan of symmetry in every class of plants, and the study of this is the foundation of every theory of natural affinities. But success in this study depends on the certainty with which organs are distinguished, and the distinguishing them must be independent of changes of form, size, and function. He then shows that the difficulties in the morphological comparison of organs, or, as we should now say, in the establishing the homology, are due to three causes; abortion, degeneration, and adherence (adhérence). These three causes, by which the original symmetry of a class is changed and may even be utterly obscured, are then fully illustrated by examples.
In respect to abortion he distinguishes that which is produced by internal causes from that which is due to accidental and external ones; he refers especially to the abortion of two loculaments in the fruit of the horse-chestnut and the oak, to the suppression of the terminal bud in some shrubs by the adjoining axillary buds, and to the fact that all organs of plants may become abortive in a similar manner; for instance, the sexual organs disappear entirely in the disk-flowers of Viburnum Opulus, and one of the two sexes in the flower of Lychnis dioica. He goes on to answer the question, how it is possible to discover the symmetry in such cases; one method he finds supplied by monstrosities, among which there are even some that may be regarded as a return to the original symmetry, the cases known as peloria. Analogy or ‘induction’ is, he says, less certain, but of much more extensive application; this is founded exclusively on the knowledge of the relative position of organs. Armed with this, we find that the flower of Albuca, which corresponds to a flower of Liliaceae in everything except in having only three stamens, is to be considered one of the Liliaceae, because it has three filaments placed between the three stamens exactly in the position of the three other stamens in the Liliaceae; it must be concluded therefore that they are abortive stamens. Similar conclusions from analogy must be carried from species to species, from organ to organ, and the great systematists have in fact done so. In certain cases abortion is produced by defect, in others by excess of nourishment, of which he gives examples. An important sentence occurs in this place; everything in nature, he says, leads us to believe that all organisms in their inner nature are regular, and that different forms of abortion differently combined are the cause of all irregularity; from this point of view the smallest irregularities are important, because they lead us to expect greater ones in nearly allied plants; and wherever in a given system of organisation there are inequalities between organs of the same name, the inequality will possibly reach a maximum, that is, end by annihilating the smallest part. Thus in the Labiatae with two stamens, it is the two which in other cases also are the smaller, which are here completely aborted. When in Crassulaceae there are twice as many stamens as petals, those that alternate with the petals are larger and earlier developed, and we may therefore expect that those which are opposite the petals may become abortive; and therefore we may place a genus like Sedum, in which the latter are sometimes wanting, with Crassulaceae; but we could not do so, if we found only the stamens that are superposed upon the petals. It occurs sometimes, he continues, that an organ is prevented from fulfilling its function by partial abortion. In this case it may assume another function, as the abortive leaves of the vetch and the abortive inflorescences of the vine are employed as tendrils. In other cases the abortive organ appears to be quite useless, as for instance many rudimentary leaves. All such useless organs, says De Candolle, exist only in consequence of the primitive symmetry of all organs. Finally the abortion may be so complete that no trace of the organ remains, of which case there are however two kinds, one where the organ is at first perceptible and afterwards quite disappears, as in the abortive loculaments in the fruit of the oak; in other instances no trace is to be seen from the first of the abortive organs, as happens with the fifth stamen of Antirrhinum.
All that has here been said might be alleged word for word in proof of the theory of descent, but our author is an adherent of the dogma of the constancy of species; what from his point of view he really means by abortion is difficult to say, for the object which is aborted is wanting. If species are constant, and therefore of absolutely distinct origin, we must not speak of abortion; we can only say that an organ which is present or large in one species is small or wanting in another. In introducing the idea of abortion De Candolle at once goes beyond the dogma of the constancy of species, without being clear in his own mind with regard to this important step. His proceeding shows that facts lead even a defender of constancy against his will to theories which run counter to that dogma. This is confirmed by his perception of the correlation of growth, which is connected with abortion; he points to the fact that owing to the disappearance of sexual organs in the disk-flowers of Viburnum Opulus the corollas become larger, as do the bracts of the abortive flowers of Salvia Horminum; similarly he regards the disappearance of the seeds in Ananas, Banana, and the Bread-fruit tree as the cause of the enlargement of the pericarps; it does not escape him, that the fertile flower-stalks in Rhus Cotinus remain naked, while an elegant pubescence forms on the barren ones; the leaf-like expansion of the leaf-stalks of Acacia heterophylla, which do not develop their laminae, he refers also to this correlation of growth. He finds the most remarkable example of the kind in the doubling of flowers, where according to his view the disappearance of the anthers is a condition of the corolline expansion of the filaments; in the same way sometimes the carpel is changed into a petal through the disappearance of the stigma. Though in many of these cases it is quite possible to conceive of the relations of cause and effect in the reverse way, yet De Candolle’s principle of correlation will be equally applicable.
The second cause by which the symmetry may be obliterated, namely degeneration, asserts itself in the formation of thorns, of threadlike prolongations of membranous expansions, and in the production of fleshy parts, or of parts with dry membranes.
The third kind of departure from the symmetrical plan is the adherence of parts, the theory of which he grounds first and chiefly on the phenomena of grafting, and then passes to more difficult cases. The close packing of the ovaries in some species of honeysuckle, is, he says, the primary cause of their adherence. This therefore does not depend on the plan of symmetry, but upon an accident, which however is constant in its appearance, owing to the specific constitution of such plants. In connection with the phenomena of adherence he next considers the question whether a structure composed of several parts, as for instance a compound ovary, should be considered as originally simple and afterwards divided into parts, or whether the converse is the true account, and he says that we must examine each particular case and decide which is the correct conception. Thus it may be shown that the perfoliate leaves of honeysuckles, as well as the involucres of many Umbelliferae, and monosepalous calyces and monopetalous corollas are due to adherence, and he proceeds to prove that ovaries with several loculaments and several parts have in like manner been formed by adherence of two or more carpellary leaves, and concludes by pointing out the systematic importance of such considerations. Further on he takes occasion to speak of the significance of the relative number of the parts of the flower, on which head he says much that is good, but does not thoroughly investigate the matter; it was not till a later time that Schimper’s doctrine of phyllotaxis made it possible to express these relations of number and position more precisely.
He concludes his rules for the application of his morphology to the determination of relations of affinity with the declaration, that the whole art of natural classification consists in discerning the plan of symmetry, and in making abstraction of all the deviations from it which he has described,—much in the same way as the mineralogist seeks to discover the fundamental forms of crystals from the many derivative forms. It is obvious that all this teaching was a great step in advance upon the right path, that De Candolle has here given utterance for the first time to an important principle of morphology and systematic botany; nevertheless he did not succeed in always consistently carrying out his own principle; he was true to himself only in the determination of small groups of relationship; in framing the largest divisions of the vegetable kingdom he entirely lost sight of the rule which he had himself laid down, that the morphological character of organs and the extent to which it can be turned to account for systematic purposes is entirely independent of their physiological character, and that the most important physiological characters are just those which are of quite subordinate importance in the determination of affinities. In spite of this strange inconsistency, to De Candolle belongs the merit of being the first to point emphatically to the distinction between morphological and physiological marks, and to bring clearly to light the discordance between morphological affinity and physiological habit; but in this discordance lurks a problem, which could only be solved forty years later by Darwin’s theory of selection. A genuine inductive process alone could reveal these remarkable relations between the morphological and physiological characters of organs. But it is at the same time true that De Candolle could not have made this discovery, if his predecessors had not already established a large number of affinities. It was while he was engaged in an exact comparison of forms already recognised as undoubtedly related to one another, that that which he called the plan of symmetry, and which was afterwards named a type, revealed itself to him; and as he examined it more closely, and compared it with peculiarities of habit in different plants formed on the same plan, he discovered certain causes, by means of which the deviations were to be explained; these were abortion, degeneration, and adherence. By attending to these he succeeded in discovering affinities that had been hitherto doubtful or unknown; this was at all events the true inductive way of advancing the system, and whatever the earlier systematists had effected that was really valuable had been effected virtually in the same way, only they never arrived at a clear understanding of their own mode of proceeding; they had followed unconsciously the method which De Candolle clearly understood and consciously pursued.
The majority of De Candolle’s successors were far from fully appreciating the entire significance of his theory, its importance as a matter of method and principle; on the contrary in the search for affinities they continued to surrender themselves to a blind feeling rather than to a clearly recognised method, and the same must be said unhappily of De Candolle himself, when he was dealing with the establishment of the large divisions of the vegetable kingdom. With equal surprise we find him in the book before us, in which he has developed the true method in systematic botany, expressing the opinion that the most important physiological characters must be employed for the primary divisions of the system, and this idea is not improved by the fact that he ascribes to the organs physiological characters which they do not really possess; thus he regards the vessels as the most important organs of nutrition, which they are not in fact, and upon this double error he builds his primary division of the whole vegetable kingdom into vascular and cellular plants, and then by a third mistake believes that this division coincides with the division of plants into those which have and those which have not cotyledons. The already established division into Monocotyledons and Dicotyledons, which rests upon a leading and purely morphological mark, is spoilt by De Candolle through his following Desfontaines in ascribing to the Dicotyledons a different mode of growth in thickness from that of the Monocotyledons, and characterising the one as exogenous, the other as endogenous. But this notion is utterly incorrect, as von Mohl showed twelve years later; and if it were correct, it would still be unimportant in a systematic point of view, because it appeals to a mark which is morphologically of quite subordinate importance. The worst consequence of these mistakes was, that the Vascular Cryptogams were introduced into the same class with the Monocotyledons,—a decided step backwards, if we compare De Candolle’s system with that of Jussieu. In spite of these grave defects in the primary divisions of the whole vegetable kingdom De Candolle’s system deserved the fame which it acquired and long maintained; it had this advantage over Jussieu’s system that in the class of Dicotyledons, the largest division of the whole kingdom, larger subdivisions appeared, and these served to unite families that were in many points essentially related; the Dicotyledons were in fact divided first of all into two artificial groups according to the presence of two floral envelopes or one; the first and much the larger of these was again broken up into a series of subordinate groups, which pointed in many ways to natural affinities. That these groups, which have only quite recently been materially altered, did to a very considerable extent take account of natural affinities, is due to the fact that De Candolle in framing them really followed his own rules, whereas the superior divisions, which are artificial, owe their existence to his disregard of them.