Chapter III (2)

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.

De Candolle declared emphatically against the old notion, that the vegetable system answers to a linear series,—a notion which sprang from a misunderstanding of the saying, ‘Natura non facit saltus,’—and demonstrated its impossibility by examples; but he allowed himself to be too much influenced by the idea which had been thrown out by Linnaeus, and taken up by Giseke, Batsch, Bernardin de St. Pierre, L’Heritier, Du Petit-Thouars and others, that the vegetable kingdom might be compared as respects its grouping to a geographical map, in which the quarters of the globe answer to the classes, the kingdoms to the families, and so on. If the theory of descent is to a certain degree compatible with the idea of a linear sequence from the most imperfect to the highest forms of plants, it is quite incompatible with the above comparison; and systematic investigation, led astray from the right path, is in danger of ascribing the importance of real affinities to mere resemblances of habit, incidental analogies, by which a group of plants appears to be connected with five or six others. In exhibiting his system on paper De Candolle allowed the use of the linear sequence as a convenience, for here it was not, he said, a matter of any importance, since the true task of the science is to study the relations of symmetry in each family and the mutual relations of families to one another; yet in a linear presentation of the system for didactic purposes the sequence ought not to begin with the most simple plants, for these are the least known, but with the most highly developed. Thus De Candolle was the means of removing from the system the last trace of anything which harmonised with an ascending and uninterrupted development of forms. Resting on the doctrine of the constancy of species, and assuming that every group of relationship is founded on a plan of symmetry round which individual forms are grouped as crystals round their parent form, De Candolle was quite consistent in his views. The mode of representation came to prevail in the vegetable kingdom which De Candolle’s contemporary, Cuvier, an equally sturdy defender of the dogma of constancy, had introduced in the animal kingdom as the type-theory. Thus the most splendid results obtained by induction were united in the case of De Candolle with the barren dogma of the constancy of species, which, as Lange wittily remarks, comes direct from Noah’s ark, to form an intimate mixture of truth and error; nor did De Candolle’s many adherents succeed in unravelling the coil, though they removed the chief errors from his system and introduced many improvements.

To these remarks may be appended a table of the main divisions of De Candolle’s system of 1819, which so far as it is presented in linear arrangement he calls expressly an artificial system.

I. Vascular plants or plants with cotyledons.

1. Exogens or Dicotyledons.
A. With calyx and corolla:
Thalamiflorals (polypetalous hypogynous),
Calyciflorals (polypetalous perigynous),
Corolliflorals (gamopetalous).
B. Monochlamydeous plants (with a single floral envelope).

2. Endogens or Monocotyledons.
A. Phanerogams (true Monocotyledons),
B. Cryptogams (vascular Cryptogams including Naiadeae).

II. Cellular plants or Acotyledons.

A. With leaves (Muscineae),
B. Without leaves (Thallophytes).

The number of families, with Linnaeus 67, with A. L. de Jussieu 100, was increased by De Candolle to 161.

If the principles of comparative morphology laid down by De Candolle were at first prevented from being rapidly disseminated in Germany by the philosophical tendencies then reigning among its botanists, and especially by the obscurities of Goethe’s doctrine of metamorphosis, yet these principles and his views also on the natural system won their way by degrees to acknowledgment and acceptance; and after the year 1830 the study of the system was prosecuted by the botanists of Germany, as well as by those of England and France, as the proper object of the science. We may even say that the impulse given by De Candolle worked more powerfully from that time forward in Germany than in France. It may be said too of De Candolle’s contemporary, the Englishman ROBERT BROWN[38] (1773-1858), whose chief labours fall in the period between 1820 and 1840, that he, like De Candolle, was better appreciated during that time in Germany than in any other country. Robert Brown, who spent the five years from 1801 to 1805 in Australia, studied the flora of that quarter of the world, and discussed in numerous essays the botanical results of various journeys made by other naturalists in polar regions and in the tropics. In this way he found opportunity to leaven the ideas, which through Humboldt’s influence had become predominant respecting the geography of plants, with the spirit of the natural system; he also made the morphology and systematic position of a number of families the subject of critical investigation.

Robert Brown’s literary efforts were limited to these monographs; he nowhere attempted to give a connected account of the principles which he follows in them, an exposition of his morphology or a theory of classification, nor did he frame a new system. The results of his studies which were really fruitful and served to advance the science are to be found in the more general remarks, which he managed to insert quite incidentally in his monographs. In this way he succeeded in clearing up the morphology of the flower and with it the systematic position of some difficult families of plants, such as the Grasses, Orchids, Asclepiads, the newly-discovered Rafflesiaceae and others, and to throw new light at the same time on wider portions of the system; in his considerations on the structure and affinities of the most remarkable plants, which had been collected in Africa by different travellers in the years immediately following 1820, he discussed difficult and remarkable morphological relations in the structure of the flower. He referred especially in this essay (1826) to the relations between the numbers of the stamens and carpels, and those of the floral envelopes in the Monocotyledons and Dicotyledons, and showed how these typical, or as he calls them in De Candolle’s phraseology, symmetrical relations were changed by abortion, while he entered at the same time into a more exact determination of the position of the aborted and of the perfect organs, in order to discover new relations of affinity. His most valuable work in this direction is a paper on a genus Kingia, discovered in New Holland in 1825; the structure of the seeds in this genus led him to seek more accurate knowledge of the unfertilised ovule in the Phanerogams generally, and especially in the Cycads and Conifers. In spite of the labours of Gärtner and the later researches of Treviranus, there was still considerable obscurity attaching to the theory of the seed, for no one had yet succeeded in referring the position of the embryo in the ripe seed to a general law. For this it was necessary to submit the ovule before fertilisation to careful examination, and Robert Brown carried out this first step to a history of development with great success; he was the first to distinguish the integuments and the nucleus in the ovule, and the embryo-sac in the nucleus, parts which Malpighi and Grew had indeed observed but had not brought out with perfect clearness. The micropyle and the hilum of the seed had not yet been properly distinguished, but had been to some extent even confounded with one another. Robert Brown showed that the hilum answers to the point of attachment of the ovule, while the micropyle is a canal formed by the integuments of the ovule and leading to the summit of the nucleus; that in anatropous ovules the micropyle lies beside the hilum, in orthotropous ovules opposite to it; that the embryo in the embryo-sac (amnion) is always formed at the spot which lies nearest the micropyle, and that the radicle of the embryo is always turned towards the micropyle,—facts which at once established the general rule by which to determine the position of the embryo in the seed and in the fruit. He also gave the first correct explanation of the endosperm as a nourishing substance formed inside the embryo-sac after fertilisation, and more than this, he was the first to distinguish the perisperm as a substance formed outside the embryo-sac in the tissue of the nucleus.

In this way Robert Brown established morphological relations in the organisation of the seed of the Monocotyledons and Dicotyledons, which count among the most important principles of classification in these classes; he was still more happy in being the first to detect the peculiar structure of the flower of Conifers and Cycads, as compared with that of other flowering plants; it was he who perceived that what had been hitherto called a female flower in these plants was really a naked ovule, a view which Trew of Nüremberg had, it is true, suggested in the year 1767. He also called attention to the agreement in structure of the male and female organs in these families. Thus one of the most remarkable facts in vegetation, the gymnospermy of the Conifers and Cycads, was for the first time established, and this led afterwards through Hofmeister’s investigations to the important result, that the Gymnosperms, which had been up to that time classed with Dicotyledons, are to be regarded as co-ordinate with Dicotyledons and Monocotyledons, forming a third class through which remarkable homologies were brought to light in the propagation of the higher Cryptogams and the formation of seeds in Phanerogams. No more important discovery was ever made in the domain of comparative morphology and systematic botany. The first steps towards this result, which was clearly brought out by Hofmeister twenty-five years later, were secured by Robert Brown’s researches, and he was incidentally led to these researches by some difficulties in the construction of the seed of an Australian genus. He discussed in a similar manner, if not always with such important results, a great variety of questions in morphology and systematic botany; even purely physiological problems were raised by him in this peculiar way, and especially the question how the fertilising matter of the pollen-grains is conveyed to the ovule; he had already concluded from the position of the embryo that it is conveyed through the micropyle and not through the raphe and the hilum, as was then supposed, and he was the first also to follow the passage of the pollen-tubes in the ovary of Orchids up to the ovules; but this is a point which will be more properly considered in the history of the sexual theory.

The peculiar character of the natural system as compared with every artificial arrangement is brought out into higher relief by Robert Brown than by Jussieu and De Candolle, and he succeeded better than any of his predecessors in separating purely morphological and systematically valuable relations of organisation from the physiological adaptations of organs. While the majority of systematists surrendered themselves to the guidance of a blind feeling in the discovery of affinities, their correct determinations being the accidental result of instinct and unconscious operations of the understanding, Brown endeavoured to give an account to himself in every case of the reasons why he took this or that view of the relationships which he determined; from what was already established and indubitable he gathered the value of certain marks, in order to obtain rules for the determination of unknown relationships. In this way he discovered also, that marks, which are of great value for classification within the limits of certain groups of affinity, may possibly prove to be valueless in other divisions. Thus Robert Brown in his numerous monographs supplied the model, by which others might be guided in further applying and completing the method of the natural system; and in this respect he was met by the botanists of Germany in the spirit of the best good-will and most profound appreciation, as is shown by the fact that a collection of his botanical works, translated by different German botanists, was edited in five volumes by Nees von Esenbeck as early as the period between 1825 and 1834. The natural system established itself in Germany through the labours of Brown and De Candolle; and the more correct appreciation of it as compared with the sexual system of Linnaeus was promoted by a work of Carl Fuhlrott which appeared in 1829, in which the systems of Jussieu and De Candolle are compared with those of Agardh, Batsch, and Linnaeus, and the superiority of the natural system is clearly set forth. A still greater effect in this direction was produced by the appearance in 1830 of the ‘Ordines naturales plantarum’ of Bartling, an independent contribution to this department of botany, and a distinct advance upon what had hitherto been effected. The contemporary monographs of Roeper on the Euphorbiaceae and Balsamineae and his treatise ‘De organis plantarum’ (1828), are an able, independent, and logical application of the principles of the morphology of the flower laid down by De Candolle and Brown to the elucidation of morphological and systematic conceptions. But the new methods of investigation introduced by De Candolle and Robert Brown had to encounter in Germany, and to some extent in France also, not only the antiquated views of Linnaeus, but, what was still worse, the erroneous notions of the nature-philosophy founded by Schelling. The misty tenets of this philosophy could scarcely find a more fruitful soil than the natural system with its mysterious affinities, and Goethe’s doctrine of metamorphosis contributed not a little to increase the confusion. These historical phenomena will be further considered in the following chapter; at present we are more concerned to show how the professed systematists pursued the path opened by De Candolle and Brown. And here it must be noticed that from about the year 1830, in Germany especially, morphological enquiry became separated as a special subject from systematic botany; it became more and more the fashion to treat the latter as independent of morphology, and thus to forsake the source of deeper insight which comparative and genetic morphology alone can open to the systematist; morphology on the other hand took a new flight, and as it thus developed itself apart from pure systematic botany, its progress must be described by itself in a later portion of this history.

If advance in systematic botany depended on the number of systems that were proposed from 1825 to 1845, that period must be looked upon as its golden age; no less than twenty-four systems made their appearance during these twenty years, without counting those which were inspired by the views of the nature-philosophy. There was great and spreading growth, but no corresponding depth; no really new points of view were opened for classification, and as regards the true principles of the natural system there were symptoms of evident decline rather than of advance, as will be shown below. Improvements were effected certainly in the details of the system, since botanists generally adhered to the principles laid down by De Candolle, Jussieu, and Brown. Families were cleared up and better defined, and groups of families were proposed which assumed more and more the appearance of natural cycles of relationship. The class more especially treated was the extensive one of the Dicotyledons, in which the families, continually growing more and more numerous, were in Jussieu’s arrangement a chaos, but had been united into larger groups in a somewhat artificial manner by De Candolle. Here we see once more how the formation of the system rises step by step from the particular to the more general; at an earlier period genera were constructed out of species, and families out of genera, and during the years from 1820 to 1845 the families were united into more comprehensive groups; but these orders or classes were not yet grouped together in such a manner as to ensure the separation of the largest divisions of the vegetable kingdom in a natural manner. The great class of Dicotyledons is not even yet so arranged that the smaller aggregates of families connect satisfactorily one with another. Nevertheless a considerable advance was made by the establishment of a large number of smaller groups of families, and Bartling and Endlicher were especially successful in founding such groups and supplying them with names and characters.

If on the other hand we turn to the primary divisions of the vegetable kingdom, we find that certain large and natural groups came to be most generally recognised and placed in the front rank in every scheme; such were the groups of the Thallophytes, Muscineae, Vascular Cryptogams, Gymnosperms, Dicotyledons and Monocotyledons. But the coordination of these great divisions of the whole vegetable kingdom was far from being rightly understood. It was usage rather than anything else, which gradually put them forward as primary types; in the systems themselves some received too great, others too little prominence, or other groups of doubtful character were admitted alongside of them. Bartling, for instance, whose system up to 1850 or even longer may rank as one of the most natural, adheres to De Candolle’s division of the vegetable kingdom into cellular and vascular plants, and rightly divides the former into two main groups, Thallophytes and Muscineae (Homonemeae and Heteronemeae), while he separates the latter into Vascular Cryptogams and Phanerogams; but the Phanerogams are divided into Monocotyledons and Dicotyledons, which again are distributed into four groups, one of these being characterised by the presence of a vitellus, that is, of an endosperm surrounded by a perisperm,—a thoroughly artificial division. The three other divisions are named apetalous, monopetalous, and polypetalous, but the Coniferae and Cycadeae are placed in the apetalous division. Less satisfactory is the primary division into Thallophytes and Cormophytes proposed by ENDLICHER[39], the latter separating into the divisions Acrobrya (Muscineae, Vascular Cryptogams, and Cycads), Amphibrya (Monocotyledons), and Acramphibrya (Dicotyledons and Conifers); the names of the three latter groups, the first of which is utterly unnatural, are founded on erroneous assumptions respecting growth in length and thickness, which Endlicher borrowed from Unger. While Endlicher’s great work has continued down to our own time to be indispensable to the botanist as a book of reference on account of the fulness of its descriptions of families and genera, the system projected by Brongniart in 1843 has acquired a sort of official authority in France. The whole vegetable kingdom is here distributed into two divisions, Cryptogams and Phanerogams, and the former are incorrectly characterised as asexual, the latter as having distinction of sex. The Phanerogams, divided into Monocotyledons and Dicotyledons, are distributed into groups in a manner that is not satisfactory; but the system has one merit, that it keeps the Gymnosperms together in one body; and if they are incorrectly classed with the Dicotyledons, it was still a sign of progress, that Robert Brown’s discovery of gymnospermy was to some extent practically recognised. The system devised by JOHN LINDLEY[40] attained to about the same importance in England as attached to those of Bartling and Endlicher in Germany, and that of Brongniart in France. After various earlier attempts he proposed a system in 1845, in which, as in Brongniart’s arrangement, the Cryptogams are characterised as asexual or flowerless plants, the Phanerogams as sexual or flowering plants; the former are divided into Thallogens and Acrogens, the Phanerogams into five classes; (1) Rhizogens (Rafflesiaceae, Cytineae, Balanophorae); (2) Endogens (parallel-nerved Monocotyledons); (3) Dictyogens (net-veined Monocotyledons); (4) Gymnogens (Gymnosperms); (5) Exogens (Dicotyledons). This classification is one of the most unfortunate that were ever attempted; the systematic value of the Rhizogens is much overrated on account of their striking habit; the Monocotyledons are separated into two classes on the strength of an unimportant mark. The characters assigned to all these groups are on the whole thoroughly faulty.

These systems have been selected for notice from among many others, because they attained an extended notoriety and importance from the circumstance that their authors, Brongniart excepted, made them the occasions of comprehensive descriptions of the whole vegetable kingdom, and again because it would be superfluous for our present purpose to bestow a closer consideration on the systems of less eminent men. Whoever desires further information on the matter will find it in the introduction to Lindley’s ‘Vegetable Kingdom’ of 1853.

If we consider the principles and points of view adopted in these systems, one thing especially strikes us, that, except in the case of Bartling, physiologico-anatomical marks were employed along with morphological ones to characterise the primary divisions; their authors fell into the mistake committed by De Candolle, and unfortunately these very marks rested in part or wholly on misapprehensions, as in Endlicher’s division into Acrobrya, etc., and Lindley’s classes of Rhizogens and Dictyogens. It was still more unfortunate that individual systematists obstinately refused to accept well authenticated facts, which it is true had not been discovered by systematists, but were nevertheless of the highest value for the system. It is scarcely credible that Lindley in 1845, and again in 1853, maintained the distinction between endogenous and exogenous growth in stems, though Hugo von Mohl had in 1831 produced decisive proof that this distinction laid down by Desfontaines and adopted by De Candolle had no real existence. The same was the case with the characters of the Cryptogams, in which the mark of having no sexual organs was repeatedly adopted as running through the whole class, although various instances of sexuality in Cryptogams were known before 1845; Schmidel had described the sexual organs of the Liverworts about the middle of the previous century, Hedwig those of the Mosses in 1782, and Vaucher in 1803 had suggested that the conjugation of Spirogyra among the Algae should be regarded as a sexual act; the systematists in fact did not know what to make of these intimations.

It was again a misfortune that the systematists in their labours often neglected to distinguish between the search for marks and the use to be made of them; the examination of all possible marks should lead to the establishing the systematic importance of certain fixed marks or their value for classification. When research has done its work, then it is sufficient in exhibiting the system to put forward only the prominent marks; and frequently a single one suffices to unite a natural group. Such a leading mark is like the standard of a regiment; its significance is not great in itself, but it serves the great practical purpose of indicating a whole group of marks which are connected with it. It was a still greater misfortune that scarcely any systematist after De Candolle endeavoured to form a clear conception in his own mind of the principles on which the natural system must be elaborated, and to set them forth in a connected form as the theory of the system. The student had to accept the arrangement offered him as a fact simply without understanding it, and the systematists themselves usually followed only a blind feeling in the framing of their groups, and never unfolded the grounds of their proceeding with logical distinctness. In this respect John Lindley forms an honourable exception, inasmuch as he did, on several occasions after 1830, give full expositions of his views on the principles of natural classification, and like De Candolle endeavoured to develop a theory of the system[41]. But he deserves credit only for the endeavour, for the principles themselves which he laid down are not only to a great extent incorrect, but they are opposed to his own and to every other natural system. We find this opposition between theory and practice much more strongly marked in Lindley than in De Candolle; the cases only are so far different, that De Candolle laid down correct principles for the determination of affinities, but in some cases did not follow them, whereas Lindley deduced quite incorrect rules of system from existing and long-established natural affinities. The consideration of all the systems framed up to the year 1853 shows clearly that the characters of truly natural groups are to be found only in morphological marks; yet Lindley enunciates the principle that a mark, or, as he incorrectly says, an organ, is more important for classification in proportion as it possesses a higher physiological value for the preservation and propagation of the individual. If this were true, nothing would be easier than to frame a natural system of plants; it would suffice to divide plants first of all into those without and those with chlorophyll, for the presence of chlorophyll is more essential than that of any other substance to the nourishment of plants, and its physiological importance is therefore pre-eminent; in that case of course such Orchideae as have no chlorophyll, the Orobancheae, Cuscuta, Rafflesia, etc., would form one class with the Fungi, and all other plants the other. It is very important for the existence of a plant whether its organisation is adapted to its growing in water, or on dry land, or underground, and if we took Lindley at his word, he would be obliged to bring the Algae, Rhizocarps, Vallisnerias, water Ranunculuses, Lemna, etc., into one group. It is very important for the existence of a plant whether it grows upright of itself, or climbs upwards by the aid of tendrils or of a twining stem or otherwise, and accordingly we might on Lindley’s principle collect certain ferns, the vine, the passion-flower, many of the pea kind, etc., into one order. It is obvious that Lindley’s main axiom of systematic botany appears in this way utterly unreasonable; yet by this principle he judges of the systematic value of anatomical characters, those of the embryo and endosperm, of the corolla and the stamens, everywhere laying stress on their physiological importance, which in these parts has really little systematic value. This mode of proceeding on the part of Lindley, compared with his own system, which with all its grave faults is still always a morphologically natural system, proves that like many other systematists, he did not literally and habitually follow the rules he himself laid down, for if he had, something very different from a natural system must have been the result. The success which was really obtained in the determination of affinities was due chiefly to a correctness of feeling, formed and continually being perfected by constant consideration of the forms of plants. It was still therefore virtually the same association of ideas as in de l’Obel and Bauhin, operating to a great extent unconsciously, by which natural affinities were by degrees brought to light; and men like Lindley, of pre-eminent importance as systematists, were, as the above examples show, never clear about the very rules by which they worked. And yet in this way the natural system was greatly advanced in the space of fifty years. The number of affinities actually recognised increased with wonderful rapidity, as appears from a comparison of the systems of Bartling, Endlicher, Brongniart, and Lindley, with those of De Candolle and Jussieu. Nothing shows the value of the systems thus produced before 1850 as classifications of the vegetable kingdom more forcibly than the fact that a clear and methodical thinker like Darwin was able to draw from them the chief supports of the theory of descent. For it is quite certain that Darwin has not framed his theory in opposition to morphology and system, and drawn it from any hitherto unknown principles; on the contrary, he has deduced his most important and most incontestable propositions directly from the facts of morphology and of the natural system, as it had been developed up to his time. He is always pointing expressly to the fact that the natural system in the form in which it has come to him, which he accepts in the main as the true one, is not built upon the physiological, but upon the morphological value of organs; it may, he says, be laid down as a rule, that the less any portion of the organisation is bound up with special habits of life, the more important it is for classification. Like Robert Brown and De Candolle, he insists upon the high importance for purposes of classification of aborted and physiologically useless organs; he points to cases in which very distant affinities are brought to light by numerous transition-forms or intermediate stages, of which the class of the Crustaceae offers a specially striking example in the animal kingdom, while certain series of forms of Thallophytes, the Muscineae, the Aroideae and others, may be adduced as instances of the same kind in the vegetable world; in such cases the most distant members of a series of affinities have sometimes no one common mark, which they do not share with all other plants of a much larger division. From these and other similar statements of Darwin we see plainly, that he actually did gather from existing natural systems of plants and animals the rules by which systematists had worked, but which they themselves observed only more or less unconsciously, and never with a full and clear recognition of them. He says quite rightly, when the investigators of nature are practically engaged with their task, they do not trouble themselves about the physiological value of the characters which they employ for the limiting a group or the establishment of a single species. Darwin clearly perceived and consistently kept in view the discordance between the systematic affinity of organisms and their adaptation to the conditions of life, which De Candolle had already but imperfectly recognised. The clear perception of this discordance was in fact the one thing needed to mark the true character of the natural system, and to make the theory of descent appear as the only possible explanation of it. The fact which morphologists and systematists had painfully brought to light, but had not sufficiently recognised in its full importance, that two entirely different principles are united in the nature of every individual organism, that on the one hand the number, the arrangement, and the history of the development of the organs of a species point to corresponding relations in many other species, while on the other hand the manner of life and the consequent adaptation of the same organs may be quite different in these allied species. This fact admits of no explanation but the one given by the theory of descent; it is therefore the historical cause and the strongest logical support of that theory, and the theory itself is directly deduced from the results which the efforts of the systematists have established. That the majority of systematists did at first distinctly declare against the theory of descent can surprise no one who observes that they were so little able to give an account of their own mode of procedure, as appears in so striking a manner from Lindley’s theoretical speculations.

One consequence of this want of clearness in combination with the dogma of the constancy of species has been already mentioned in the introduction; namely, the notion professedly adopted by Lindley, Elias Fries, and others, that an idea lies at the foundation of every group of affinities, that the natural system is a representation of the plan of creation. But the question, how such a plan of creation could explain the strange fact that the physiological adaptations of organs to the conditions of life have nothing at all to do with their systematic connection, was quietly disregarded; and in fact the notion, founded on Platonic and Aristotelian philosophy, of a plan of creation and of ideal forms underlying systematic groups, could not explain this discordance between morphological and physiological characters. It would be easy to maintain the view of the systematists, that the natural system represents a plan of creation, if physiological and morphological characters went always truly hand in hand, if the adaptation of the organs to the conditions of life in the species were perfect; but facts show that the adaptation is in the best of cases comparatively imperfect, and that it is in all cases brought about by the accommodation to new requirements of organs which originally served to other functions.