The true principles of classification, however much they may have been amplified and refined upon, were in reality expressed by Ray, when he defined a Natural System to be that which neither brings together dissimilar species, nor separates those which are nearly allied. However much the words of this definition may have been varied, it still retains the very meaning given to it by its author. A species, said Jussieu, consists of individuals very much alike in all their parts, and retaining their resemblances from generation to generation. Those species are to be associated which correspond in the greater number of their characters; but one constant is of more importance than several inconstant characters. On these two axioms hangs the whole principle of Natural classification.-(Genera Plantarum Præf.) And then he proceeded to show how a group of species combined upon this principle forms a Genus, of Genera an Order, and of Orders a Class; the same rules of combination being observed throughout, with this difference only, that the larger the group the fewer the characters by which it is limited (Quò generalior enim extat plantarum ordinatio quælibet, eò paucioribus utitur signis definientibus).

But it is far more easy to lay down principles than to put them in execution. The definition of Ray is perfect, but its application is surrounded with difficulty. The very first point to settle in attempting to carry out his views is by what rule the dissimilarity or alliance of species is to be determined. In fact, very different ideas of likeness or unlikeness are entertained by different observers. The common people can see no difference of moment between a Daphne, and a Cherry, and a Rhododendron, but call them all Laurels, although a Botanist fails to perceive their resemblance. On the other hand, there seems to the vulgar eye no connection between the Hemp plant and the Mulberry tree, and yet the Botanist brings them into close alliance. Nor are these conflicting views confined to the ignorant and the uneducated; such differences of opinion may be found among Botanists themselves. For instance, Linnæus joined Arum with Phytolacca under his Piperitæ, and Convolvulus with Viola under his Campanacei, combinations which modern Botanists entirely repudiate; and in like manner the association of Hugonia with Chlenads by Endlicher, of Nepenthes with Birthworts by Brown, of Planes with Witch Hazels by Adolphe Brongniart, of Vines with Berberries by the Author of this work, of Spurgeworts with Heathworts and Chenopods by Fries, are so many modern instances of peculiar views from which other Botanists withhold their assent.

It is therefore of the first importance to settle with something like precision what it is that constitutes likeness among plants, or, as it is technically called, their affinity.

The reason why the vulgar commit mistakes in judging of natural affinity is, because they draw their conclusions from unimportant circumstances, the chief of which are size, form, and colour. The similitude of size gave rise to the old notion that all trees made a class by themselves; which is as if in a classification of animals the horse, the lion, and elephant were placed in a different part of the animal kingdom from the rat, the cat, and the goat. Form is another of the false guides which lead to error; if all round-leaved or square-stemmed plants are to be associated, so ought glass to be classed with the diamond when it is cut to the same shape. Colour is less a source of mistake, and yet it is sometimes unconsciously employed by the superficial observer, as when he calls all yellow-flowered Composites Marigolds, and all white-flowered vernal bushes Thorns. It

must be evident to the most careless thinker that such resemblances are trifling.

That which really determines affinity is correspondence in structure. It may be said that.those plants are most nearly related which correspond in the greatest number of points, and those the most distantly in which we find the fewest points of correspondence; and this must be true when we remember that if every point in the structure of any two plants is found to be alike, then those two must be identical. But it will be obvious that an examination of all plants through every detail of their organisation is impracticable; it has never in fact been accomplished in any one case. Experience must have shown that the organs of vegetation are of very different degrees of value in determining resemblance in structure, that some are of paramount importance, others of less consequence, and others of comparative insignificance. Hence the relative value of characters forms a most important part of the study of the Botanist; it is in fact the pivot upon which all the operations of a systematist must turn.

The only intelligible principle by which to estimate their respective value is according to their known physiological importance; regarding those organs of the highest rank which are most essential to the life of the plant itself; placing next in order those with which the plant cannot dispense if its race is to be preserved; assigning a still lower station to such organs as may be absent without considerable disturbance of the ordinary functions of life; and fixing at the bottom of the scale those parts, or modifications of parts, which may be regarded as accessory, or quite unconnected with obviously important functions.

The first office which all organised beings have to perform is that of feeding; for it is thus only that their existence is maintained. The second is that of propagating, by means of which their species is perpetuated. These being functions of the highest importance, it is reasonable to conclude that the organs provided for their proper execution must be of the highest importance also, and hence that they are beyond all others valuable for the purposes of classification. And, again, because the power of feeding must come before that of propagating, it might be conjectured beforehand that the organs destined for the former operation would afford the first elements of a Natural method. But since the action of feeding is very simple in the Vegetable Kingdom, because of the similar modes of life observable among plants, while, on the contrary, the act of propagation is highly diversified, on account of the very varied nature or structure of the parts by which it is accomplished; so might we conjecture that the organs of nutrition would afford but few distinctions available for purposes of classification, while those of fructification would furnish many. And such is the fact. Hence it is that the great classes of plants are principally distinguished by their organs of growth, and that in the numerous minor groups such peculiarities are comparatively disregarded, their chief distinctions being derived from their parts of reproduction. These principles are more fully expressed in the following axioms:

1. Peculiarities of structure which are connected with the manner in which a plant is developed are physiological; those which are connected with the manner in which parts are arranged are structural. Physiological characters are of two kinds, viz., those which are connected with the mode of growth (the organs of vegetation), and those which regulate reproduction (the organs of fructification). Physiological characters are of greater importance in regulating the natural classification of plants than structural.

2. All modifications of either are respectively important, in proportion to their connection with the phenomena of life.

3. If we allow ourselves to be steadily guided by these considerations, we shall find that the internal or anatomical structure of the axis, and of the foliage, is of more importance than any other character; because these are the circumstances which essentially regulate the functions of growth, and the very existence of an individual.

4. That next in order is the internal structure of the seed, by which the species must be multiplied. Thus the presence of an embryo, or its absence, the first indicating a true seed, the latter a spore, are most essential circumstances to consider. And so also the existence of albumen in abundance round the embryo, or its absence, must be regarded as a physiological character of the highest value: because, in the former case, the embryo demands a special external provision for its early nutriment, as in oviparous animals; while, in the latter case, the embryo is capable of developing by means of the powers resident in itself, and unassisted, as in viviparous animals.

5. Next to this must be taken the structure of the organs of fructification, by whose united action the seed is engendered; for without some certain, uniform, and invariable action on their part, the race of a plant must become extinct. Thus we find that the structure of the anthers, placentæ, and ovules, are more uniform than that of the parts surrounding them, while their numbers are variable; and the condition of the filament, which appears of so little importance in a physiological point of view, is also inconstant. So also the texture and surface and form of the pericarp, which acts as a mere covering to the seeds, is not to be regarded in these inquiries, and, in fact, differs from genus to genus; as, for instance, between Pyrus and Stranvæsia, or Rubus and Spiræa, in the truly natural Rosaceous Order.

6. On the other hand, the floral envelopes seem to be unconnected with functions of a high order, and to be designed rather for the decoration of plants, or for the purpose of giving variety to the aspect of the vegetable world; and, consequently, their number, form, and condition, presence or absence, regularity or irregularity, are of low and doubtful value, except for specific distinction. There seems, indeed, reason to expect that every Natural Order will, sooner or later, be found to contain within itself all the variations above alluded to. Even in the cases of regularity and irregularity we already know this to be so; witness Veronica and Scoparia in Figworts, and Hyoscyamus in Nightshades, Delphinium in Crowfoots, and Pelargonium in Cranesbills.

7. The consolidation of the parts of fructification is a circumstance but little attended to in a general point of view, except in respect to the corolla ; but as it seems to indicate either the greatest change that the parts can undergo, or, where it occurs between important and usually unimportant organs, that in such cases the latter become essential to the former, it probably deserves to be regarded with great attention. For instance, the presence or absence of the corolla is often a point of little moment, and is, we know, a very fluctuating circumstance. This is especially true of those Natural Orders in which the stamens and petals are separated; as in Roseworts, Rhamnads, Onagrads, &c. On the other hand, when the stamens, which are indispensable organs, adhere to the petals, the latter are more constantly present, as in Figworts, Acanthads, Nightshades, &c.

There are also certain other principles which experience tells us the systematist must keep in view; and most especially that of regarding of

importance whatever appears to be constant in its nature among nearly allied species. Nothing which is thus constant can be considered unimportant, for everything constant is dependent upon or connected with some essential function. Therefore all constant characters, of whatever nature, require to be taken into account in classifying plants according to their natural affinities. Of this nature are the internal structure of stems and leaves, the anatomical condition of tissue, the organisation of the anther, pollen, and female apparatus, and the interior of the seed.

On the other hand, whatever points of structure are variable in the same species, or in species nearly allied to each other, or in neighbouring genera, are unessential to the vital functions, and should be set aside, or be regarded as of comparative unimportance. Hence the badness of the Monopetalous, Polypetalous, and Apetalous divisions of Jussieu, depending upon the mere presence or absence, and union or disunion, of petals. The genus Fuchsia, for example, has petals highly developed; but in F. excorticata they are absent, and yet the plant differs no otherwise from the rest of the genus the same is true of species of Rhamnus. Again, the Rue has the petals separate; and Correa, very nearly allied to it, has them combined.

All classifications in which the foregoing principles are observed are natural; and that will be the most stable in which they are employed with the greatest skill. Some writers, indeed, maintain that there cannot be more than one really natural system, any more than one planetary system; and in a certain sense this may be true, inasmuch as we must suppose that one plan only has been observed in the creation of living things, and that a natural system is the expression of that plan. But, on the other hand, it must not be forgotten that such a plan may be represented in many ways; and that although the order of nature is in itself settled and invariable, yet that human descriptions of it will vary with the mind of the describer. A universal history is a collection of events; but it is not necessary that all universal histories should follow the same order of narration. The events themselves are unalterable, but the way of combining them and causing them to illustrate each other is manifold.

In natural science, indeed, the mode of arranging the matter is susceptible of infinitely more variation than history: because in the latter subject time is an inflexible leader who cannot be lost sight of. But in natural science there is no beginning and no end. It is impossible, from the nature of things, that any arrangement should exist which shall represent the natural relations of plants in a consecutive series. It is generally admitted by those who have turned their attention to a consideration of the manner in which organised beings are related to each other, that each species is allied to others in different degrees, and that such relationship is best expressed by rays (called affinities) proceeding from a common centre (the species). In like manner, in studying the mutual relationship of the several parts of the Vegetable Kingdom, the same form of distribution constantly forces itself upon the mind; Genera and Orders being found to be apparently the centre of spheres, whose surface is only determined by the points where the last traces of affinity disappear. But although the mind may conceive such a distribution of organised beings, it is impossible that it should be so presented to the eye, and all attempts at effecting that object must of necessity fail. If in describing the surface of a sphere we are compelled to travel in various directions, continually returning back to the point from which we started; and if in presenting it to the eye at one glance we are compelled to project it upon a plane, the effect of which is to

separate to the greatest distance some objects which naturally touch each other; how much more impossible must it be to follow the juxtaposition of matter in treating of the solid contents of a sphere!

An arrangement, then, which shall be so absolutely correct an expression of the plan of nature as to justify its being called the Natural System is a chimera.* All that the Naturalist can do is to carry into effect the principles above explained, with a greater or less amount of skill; the result of which will be a Natural System.

When Linnæus attempted to form a Natural System, he merely threw together such genera as he knew into 67 groups, which he called Fragments, and which were equivalent to the Natural Orders of Modern Botany. Jussieu advanced a step further, by forming 15 Classes, under which he placed 100 Natural Orders. At a later period the name Class was reserved for the three great divisions of Acotyledons, Monocotyledons, and Dicotyledons; and the Orders were collected into smaller groups called Sub-classes; and thus, by degrees, the necessity of forming three grades of distinctive characters superior to genera was recognised. But our countryman, Dr. Robert Brown, whose sagacity is not the least remarkable part of his scientific character, long ago pointed out the insufficiency of even this amount of subdivision, and proposed the combination of Natural Orders into groups intermediate between Orders and Sub-classes. The necessity of this measure is now universally acknowledged; attempts have been made for some years, by various Botanists, to work out the problem; and I think it must be conceded that a real advance has thus been made, by the efforts of various independent observers, to the accomplishment of so very desirable an object. To such attempts the present work is an addition.

The leading idea which has been kept in view in the compilation of it has been this maxim of Fries: Singula sphæra (sectio) ideam quandam exponit, indeque ejus character notione simplici optimè exprimitur. I cannot but think that the true characters of all natural assemblages are extremely simple; nothing can be more certain than that their value diminishes in proportion to their complexity. If two objects are not to be distinguished by a few simple circumstances, they can hardly be called distinguishable at all. In the highest groups or classes it is always so, (see p. 4;) and there is no apparent reason why the same rule should not obtain in groups of a minor rank. Nevertheless, we find that this is too often lost sight of, and that long details of structure are substituted for precise words of distinction.

It may be, and certainly is in some measure, true, that insuperable difficulties are, in the present state of our knowledge, opposed to strict definitions of Natural Orders, and à fortiori of their Alliances, &c. But that is no reason why we should not endeavour to render their distinctive characters as precise as the nature of the subject will permit. Vague distinctions, which are at once the bane and opprobrium of Natural History, are so repulsive to the understanding as to deter the mass of mankind from giving it their attentive study. And it is not too much to assert that this vagueness arises more frequently out of the prejudices or mistiness of the Naturalist's own mind than out of things themselves. It will constantly happen that two groups may stand, by common consent, in the nearest conceivable relation to each other; it is quite possible, by one way of arranging

* Systema illud naturæ ipsius absolutum (quod mera empiria captant!) mens humana capere nor potest; est quoddam supra naturale cujus clavem, manibus v. ingenio humano non prensandam, summus tantum tenet Naturæ auctor.-Fries Corpus Florarum, p. xvii.