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.