There can be no doubt that amongst a large class of ordinary observers a clear perception of the generic system, in an abstract sense, does not by any means prevail. What the nature of a genus really is, would appear to have been very commonly overlooked, or perhaps misunderstood, by people of this stamp; and the consequence has been, that the wildest notions have frequently arisen, even from men of sound specific attainments, as to the claims (for annihilation or retention, as 'genera') of certain subsidiary zoological assemblages. The terms 'genus' and 'species' have been conjointly so long associated in our minds with the selfsame things (whatsoever they may be), that they have become almost part and parcel of the objects themselves; so that the student who does not sufficiently reflect on their true signification, is apt to regard them as of equal importance,—or, rather, more often perhaps than otherwise, to make the latter subservient (or inferior) to the former! This however is, in reality, the very reverse of what should be the case, as a moment's consideration will indeed at once convince us: for what are genera, after all, but dilatations (as it were) along a chain which is itself composed of separate, though differently shaped, links? The links (or the actual, independent bodies which constitute the chain) are the species; but the knobs, or swellings, which their several forms may tend, by degrees, to establish along its course (through the slight disparity which each of them presents from that which is next in succession to it; and therefore through the gradual manner in which the bulbs, or nodules, may be said, on the whole, to be produced), are the groups into which those species naturally fall. It matters not a straw whether these assemblages be primary, secondary, tertiary, &c.,—in other words, whether they be departments, families, or genera, as usually understood,—the principle is in every instance the same; the difference being merely relative, and not absolute.
Or, if we choose to vary the simile, we may compare the whole system to a cord, upon which beads, of innumerable sizes, patterns, and colours, have been densely strung. Now, if there were no such things as natural divisions in the organic world, these beads (which represent the separate species) might have been disposed of anyhow,—their positions, with respect to each other, would under those circumstances have been of no importance. But such is not the case: there is an order and method throughout Nature, which shows that every individual portion of it has been adjusted by the Master's hand, and that nothing has been left to chance. Those beads (to follow up the metaphor) of countless magnitudes and hues, have had their proper places allotted to them,—and moreover with such care and regularity, that a complete plan, or scheme, of distribution is at once conspicuous. Although there are not even two, amongst that enormous multitude, which are precisely alike (for every species, however it may resemble its next ally, has some distinctive feature of its own), we immediately perceive that those beads which have most in common, are, as it were, attracted to each other,—so as, by their close approximation, or contact, to create excrescences and stripes, of divers kinds, along the entire length of the cord. If we assume now that the red beads have been collected together, to the length (for instance) of a yard, and that within that space a dozen protuberances, of discordant aspects and dimensions, have (by the union of those beads which more nearly simulate each other) been brought about; we shall have a very fair idea of the ordinary grouping of the animate tribes. The red beads, taken in the mass, may be likened to a perfect "family;" the differing gibbosities to twelve well-marked "genera," which that family includes; whilst the "species" (the real dramatis personæ, of independent existence, which are nevertheless compelled to occupy the situations we have described,—thus causing the divisions to be mapped out) are here typified, as everywhere, by the several beads themselves.
I have not thought it necessary to pursue this reasoning into higher divisions than "families;" but of course it may be extended to any amount,—so as to shadow forth, equally, the compartments of primary significance. Nor would I wish to imply, by the above similes, that I regard a lineal method of arrangement as the correct one. Every zoologist is aware, that in Nature such does not exist: but the mode of illustration which I have selected is applicable to all systems alike, so far as the principle is concerned.
It will consequently be seen, from what has been said, that the terms "genus" and "species" not only differ very considerably in importance, but in signification also. Whilst the former is merely suggestive of a particular position which a creature occupies in a systematic scale (a position, however, which depends upon the various structural peculiarities which it possesses in common with other beings,—which thus more or less resemble it); the latter expresses the actual creature itself: so that while one applies to several animals (of distinct natures and origins, though bound together by a certain bond of imitation), the other belongs to a single race alone, which it therefore exclusively indicates. But if such be the case, it will perhaps be asked,—Why then insist upon a generic name at all, if the specific one be sufficient to denote all that is required, namely, the animal itself? To which, however, we may reply, that the binomial nomenclature is demanded for two elementary reasons,—first, because it is founded upon a natural truth, which (to say the least) it would be unwise to violate; and, secondly, because it is convenient, both for simplification and analysis. We should assuredly be surprised were a man to object to his surname, as unnecessary, because he has a christian (or specific[79]) one which is the exponent of him alone. True it is that his family (or generic) title applies to the rest of his kin also; but, since there are other people (of other families) who may have the same individual appellation as himself, it is clearly desirable, even as a matter of expediency alone, that patronymic and christian name should be alike retained. We need not, however, plead expediency, in favour of this acceptance of what has been so long tested, and shown to be correct; we appeal to a higher tribunal,—that of experience,—in proof that it draws its origin from Nature itself, and is implied by the very existence, or reality, of natural groups. The 'Méthode Mononomique' has indeed been attempted[80]; and it has failed,—or at any rate it has shown itself to be inferior, both ideally and in practice, to the plan commonly in use: and if I might be pardoned a passing conjecture on its ultimate success, I should be inclined, since it is contrary to the canon of the organic world, to regard its case as utterly hopeless.
Let us not be unfair, however, towards those who have sought to establish a nomenclature which they conceived would be less open to objections than that which we have been hitherto accustomed to endorse. The notion did, at any rate, arise out of an apparent defect in the binomial process,—for the inconveniences which they complained of are real ones; and, having felt them practically, they aspired to sweep them away by remodelling the whole system afresh. But, had it not been for an evident misconception of the generic theory, in the abstract, the trial would in all probability have never been made; and we should have been spared the downfall of a contrivance which has had but little to recommend it beyond the ingenuity of its machinery and detail. If we analyse the motives for this experiment, we shall find that it originated from a belief, that genera are either purely imaginary, or else that they must (like species) have a definite and isolated existence. Now both of these conclusions appear to be equally gratuitous and untenable; and such as a lack of observation could alone beget. Genera are not mere phantoms of the brain (as most naturalists will readily admit); but they are, likewise, by no means abrupt, or well-marked, on their outer limits (except indeed by accident,—of which hereafter), but merge into each other by gradations, more or less slow and perceptible. Such being the case, we can easily understand why it is that the followers of the 'Méthode Mononomique' (who, paralysed by the fact that genera are seldom clearly defined at their extremes, would seem to repudiate them in toto) have rashly regarded the binomial system as intolerable. Finding that it was possible for numerous species, whose structural characteristics were less conspicuously pronounced than those of their allies, to be enumerated, and with equal plausibility, under two consecutive groups; they immediately inferred that the groups themselves could not be upheld on account of these connective links: and so it was resolved (through a new and artificial scheme) to ignore them; and to fall back upon the creed, that species alone (and not genera) are to be recognized in the organic world. This was but the device, however, at the outset, of a single mind; and the perverts to it have been but few. It is in direct opposition to the first principles of nomenclature, and sets at defiance a great natural truth.
But what, it may be inquired, is this great primary truth which the monomial system tends to violate? I repeat what I have already stated, that it is the existence of natural assemblages which that scheme would, if it were practicable, discountenance. Order and symmetry, however (which involve classification, or arrangement), are the law of Nature, and it is not possible to set them aside. It matters not if harsh lines of demarcation are undiscernible between the several consecutive groups,—the groups themselves must still remain (however equivocal it may be where they exactly commence or terminate), and cannot be wiped out. To suppose à priori that the allied divisions of the animate creation are perfectly disconnected inter se, is in fact to break the chain on which the unity of the organic world depends; whilst to assume that groups cease to be groups when they can be discovered to merge into each other, would no less destroy the harmony of that admirable method, or array, which the naturalist, above all others, delights to contemplate. If things are no longer to be regarded as dissimilar because they unite on their outer limits, differences may be given up, as having no special meaning, and as therefore unworthy of investigation. It requires but a slight insight into the physical universe to be convinced, that nearly everything which we see (and, moreover, without injuring its individual reality) is blended into that to which it is the most akin. Night is distinct from day; yet, so long as the twilight intervenes, no man can pronounce where the one ends, and the other begins. Heat is opposed to cold; yet, if by degrees they be respectively diminished, they will at last amalgamate, in a central temperature. And thus it is with things material. The sea and the land are essentially unlike; yet the precise boundary between the two is never clearly defined,—the ebb and flow are constantly going on, and the line of separation is variable. The mountain-range is moulded on a different type to the level country beneath it; yet the turning-point of them both is, in all instances, on neutral ground. We need not however adduce further evidence in support of this fact,—that, throughout the whole of Nature, the general principle of fusion (either absolute or apparent) is most obvious. From first to last, traces of it are everywhere to be detected; not only between clusters, or material combinations, of objects (in which case it is absolute), but even between the objects themselves,—under which circumstances, however, it is merely apparent; for, since they are specifically dissimilar, it can only arise from their near resemblance to each other, and not from their positive coalescence. But, admitting that this universal blending, throughout the animate world, does not interfere with the gradual conformation of its several groups, which therefore should be recognized; we may perhaps be told by the believers in the 'Méthode Mononomique,' that they do not intend to ignore the arrangement which Nature has so broadly laid down, but that, on the contrary, they tacitly endorse it,—their device having reference to the names only. To this however it will be sufficient to reply, that, if they deem it necessary (of which I am by no means convinced) to accept the natural genera of the organic creation at all, why not acknowledge them? and how can they be so well acknowledged, either in principle or practice, as through the medium of a binomial nomenclature? Such a system is the only consistent one, on the hypothesis that they do consider them of primary importance; it is more in unison with our notions of what ought to be; more suggestive of what actually is; more honest and generous to those who have laboured (as describers), with such care and diligence, before us.
It will be perceived, from the above remarks, that, although professedly criticizing the 'Méthode Mononomique,' into the analysis of which my subject has unintentionally drawn me, it is the absurdity of objecting to genera because they are not rigidly defined throughout, that I have been mainly striving to condemn. It is indeed well nigh incredible that any such strictures could ever have been advanced; for it must surely have occurred to the most superficial inquirer, that genera, after all, cannot be homogeneous,—seeing that they are necessarily composed of detached species, no two of which are precisely similar, even in the few structural details which may have been accidentally chosen for generic diagnostics. How is it possible, therefore, that mere groups, even though they be in accordance with Nature, should be so far isolated and uniform in their character as to occupy an analogous position to that of the absolutely independent species (of distinct origins) which they severally contain?
Taking the preceding considerations into account, the question will perhaps arise,—How then is a genus to be defined? To which I may reply that, were I asked whether genera had any real existence in the animate world, my answer would be that they undoubtedly have,—though not in the sense (which is so commonly supposed) of abrupt and disconnected groups. I conceive them to be gradually formed nuclei, through the gathering together of creatures which more or less resemble each other, around a central type: they are the dilatations (to use our late simile) along a chain which is itself composed of separate, though differently shaped links,—the links being the actual species themselves, and the swellings, or nodes, the slowly developed genera into which they naturally fall. When I say "slowly developed," my meaning may possibly require some slight comment. It is simply therefore to guard against the fallacy, which I have so often disclaimed, that genera are abruptly (or suddenly) terminated on their outer limits, that the expression has been employed. Though I believe that a series of species, each partially imitating the next in contact with it, is Nature's truest system; yet we must be all of us aware that those species do certainly tend, in the main, to map out assemblages of divers phases and magnitudes, distinguished by peculiar characteristics which the several members of each squadron have more or less in common. So that it is only in the middle points that these various groups, respectively, attain their maximum,—every one of which (by way of illustration) may be described as a concentric bulb, which becomes denser, as it were, in its successive component layers, and more typical, as it approaches its core.
If, then, the theory of genera be such as I have endeavoured to expound, it results from what has been said, that every generic type is to be looked for in, or about, the centre of its peculiar group,—or at any rate in that region of it which would seem to be the most characteristically, or evenly, pronounced. I lay particular stress upon this conclusion, because (if correct) it will somewhat modify the notions which are occasionally entertained upon the subject. A stricture, however, may here be required upon what I have advanced, lest, through using the metaphors which I selected for the elucidation of a principle, it be supposed that I would wish them to apply to the smaller details, likewise, of the problem. If a genus has been portrayed under the similitude of a bulb, or of a nodule (formed by the approximation of beads which more or less resemble each other in their primary aspect), it does not follow that either bulb or nodule are to diminish in a similar ratio towards their respective circumferences,—or, which is the same thing, that they are to be symmetrical; whether spherical, ovoid, or otherwise. The general method of the organic creation is a progressive one; and its successive types, therefore, will not always be found to radiate equally from their normal foci: so that it is in the direction of the higher (rather than the lower) extremities of the assemblages that those foci are usually to be discerned;—and where the groups are large, it is not often difficult to pronounce which of their ends are, as a whole, the more perfectly developed.
It will, moreover, be further acknowledged (if my premises are allowed), that, since it is a somewhat central position which the typical member of a genus usually occupies, the diagnostic characters, although (in combination) carried out to the full, are more evenly balanced in a generic type than in any of its associates; or, in other words, that a species in which any single organ is monstrously enlarged, at the expense of the rest, is seldom typical of the assemblage with which it is placed; but may be à priori regarded as in all probability a transition form, leading us onwards into some neighbouring group[81].