First, then, let us consider affinity, which, according to the views I hold, is inseparably connected with the doctrine of types.
All organised matter is, of course, intrinsically allied in its nature, as contradistinguished from that which is not organised; this, therefore, is the first, or, as some would rather say, the last, the ultimate, the slightest possible, degree of affinity. Next, we have a grand primary distribution of all organic matter into the animal and vegetable kingdoms; a division too obvious to be for a moment called in question, and universally allowed; admitted even, inconsistently enough, by those who hold that every natural assemblage of species, great or small, forms part of some quinary circle.3
Now, I cannot but observe here, in passing, that, to any unbiased person, I should think that a due consideration of this first binary distribution must at once carry conviction to the mind, must be at once a most unanswerable argument against all quinary or similar doctrines; the which, of course, if based upon sound theory, would not only be found to hold good, but would be most obviously indicated by these primary and comprehensive assemblages of every created species.
But, to return: here we have the animal type, and the vegetable type, diverse in structure, distinct even in chemical composition, insomuch that the kingdom to which any dubious production appertains may be decided by chemical analysis, even in a fossil, should but a very few particles of its primitive substance have been preserved. Say not, that the kingdoms blend at their ultimate extremities; for there are no better grounds for this supposition than those which led many, for a time, to advocate the spontaneous generation of Infusoria; extreme minuteness alone setting the limit to a definite partition.
We must therefore admit, that there is a degree of physiological affinity between the most dissimilar animals, and also between the most dissimilar plants, which no animal or vegetable can possibly have for each other: species from the two kingdoms, however these may undoubtedly approximate at the extreme boundaries, can have no higher degree of affinity for each other than what they possess in common, as opposed to all unorganised matter; what further relations they may show are, therefore, totally distinct from affinity.
Leaving plants, we now enter upon the primary divisions of the animal creation, the separate leading types, the distinct plans, upon one or other of which all animals are organised, and which cannot, any more than the last, be confounded, in any instance, one with another, however in particular cases these too may approximate; of which presently.
Every vertebrate animal is, therefore, allied to every other vertebrate animal by what, to specify by numbers, may be expressed as three degrees of affinity; whereas it is physiologically related to every member of the Annulosa, and other invertebrate classes, by only two degrees, its affinity with plants being reckoned as one; the proportions of these numbers towards each other pretty accurately denoting the value of these degrees; two being double one, three exceeding by half two, etc. Animals of the same subclass, as different mammifers, or birds, or reptiles, are, of course, related to each other by four degrees of affinity; those of the same order by five, and so on; the number of these several degrees increasing in proportion to the number of subordinate successive types upon which different species are alike organised, and of which, successively, they are modifications, not combinations of different ones, in the last case any more than in the first.
Every modification of every successive type is thus rudimentally different from the most approximate modifications of every other equivalent type, or superior type, to which it does not appertain; and this is the same conclusion to which I have been irresistibly led from consideration of various phenomena connected with the changes of plumage which take place in birds.
As every species is perfectly and essentially distinct and separate from every other species, so, except in a retrograde direction, are the successive typical and subtypical plans upon which they are severally organised, however similar the latter may in some instances be, as are also the former. It is unnecessary to enter here upon any remarks on hybrids, as further elucidatory of the precise nature of affinity: it is well known that these can only be produced within a certain physiological range, and that their degree of fertility (paired with individuals of pure blood) is in proportion to the degree of affinity between the parent species.
By the term approximation, I must be understood to signify those modifications of particular types, which, adapted to intermediate modes of life, very commonly more or less resemble (in consequence of this adaptation) species which are organised on other and different types. I have already had occasion to mention certain extreme modifications of the corvine or omnivorous type of perching birds, which are close approximations towards the fringillidous type (as Aglaius and other finch-like Sturnidae, Ammodramus, and Alauda); the true affinities, however, of all which are at once shown by a reference to their moulting.
The hag, the lamprey, and the pride, are, thus, extreme approximations of the general vertebrate type of organisation towards the class Mollusca. The Ornithorynchus, among mammifers, approximates very remarkably towards birds; but it exhibits less analogy with them, collectively, than many rodent species do. The pronghorned antelope is an approximation in its genus towards the Cervidae; but its affinity to the latter is not greater than in other antelopes. The frigate bird is an approximation towards the eagles; yet no one would consider it as organised upon the falcon type: so the Pterocles is an approximation towards the pigeons, and the Nicobar pigeon towards the Gallinidae; each being at once referable to its particular type, though in certain adaptive relations they are intermediate.
The pipit genus is a most striking approximation of a very marked type (subordinate to the dentirostral) towards the larks; but its moultings at once intimate its true position in the system, however its general aspect might, at first sight, render this doubtful. It is by no means nearly allied by affinity to Alauda; and I will unhesitatingly venture to assert, that by no art could they be induced to unite to the production of a hybrid.
Analogy, in the most definite signification of the term, is well exemplified in the close resemblance between the mouth of the swift, and those of the larger high-flying insectivorous bats (Vespertilio). It is exhibited wherever species that are modifications of diverse types are organised to perform nearly the same part in the general economy of nature; which latter by no means necessarily implies approximation; as may be illustrated by adducing the vultures among birds, and the dog kind among quadrupeds, or certain of the Sphingidae from amongst insects, as compared to the Trochilidae of the feathered race.
It is well exemplified by the deadly spring of the cats, as compared with that of the crushing serpents, and as somewhat contradistinguished from that of the saltatory spiders; all the energy of the body being, in the former cases, remarkably concentrated in a single spring, from which exhaustion follows, while in the latter case it is not. It is curiously shown by a fact related by Sir W. Jardine, of the European howler or eagle-owl (Bubo europaeus), in which the analogy of that genus to the cat family is even more strikingly indicated than by the very remarkable general resemblance in their external aspect.
"This bird," observes Sir William, "evinces a great antipathy to dogs, and will perceive one at a considerable distance; nor is it possible to distract its attention so long as the animal remains in sight. When first perceived, the feathers are raised," etc., exactly as a cat raises her fur at sight of her natural enemy; though, in either case, it is difficult to say why they should be inimical. No doubt, however, the purpose, the reason for this antipathy, is the same in both instances, and it is for the naturalist to endeavour to find it out.
The common pipit, a modification of the dentirostral type; and the Lapland snowfleck, one of the conirostral (as here limited); are in so far related to each other by analogy, as that they are both approximations towards the lark genus, an extreme modification of the omnivorous or corvine type; they are therefore related to each other by a certain analogy; to Alauda, by approximation; and to all the members of their respective separate groups, by an additional degree of affinity to what subsists between either of them and the others. Affinity and analogy, of course, coexist, as all organisms are, at least, related by what I have termed the first degree of the former; but the extent of the former does not necessarily affect that of the latter: vultures and dogs, for instance, are allied by three degrees of affinity; while the carrion beetles (Carabidae) are related to either by only two degrees: yet the analogy is as great in the one instance as in the other.
Pure analogy may subsist with very trifling approximation; as is shown by the already cited case of the cats and serpents, or as may be exemplified by a hundred similar in stances of corresponding groups existing in major divisions of diverse structure, in which, however marked the analogy, however similar the office they were destined to perform the degree of approximation is in many instances quite imperceptible.
Affinity, approximation, and analogy, may therefore be collectively defined as pertaining to the physiological relations subsisting between different species, as opposed to their adaptive relations; of which latter they are wholly independent: that is to say, different species, nearly allied by either of these physiological relations, exhibit no mutual, no relative adaptation towards each other's habits and structure; such as we observe in the huge claws of the anteater (Myrmecophaga), evidently furnished in direct relation to the habits of a particular group of insects, the mounds of which they are obviously intended to scrape open, while the tongue is as expressly modified to collect the aroused inhabitants, upon which alone the creature is fitted to feed, and upon the supply of which, therefore, as an existing species, its being altogether depends.
Adaptive relations are, in general, even more obvious and striking in groups which are physiologically the most widely removed; as may be exemplified by adducing the bill of the crossbill, modified in direct relation to the seminiferous cones of the Coniferae; or the recurved bills of certain humming birds, to the bent tubes of the corollas of particular Bignoniaceae, etc. Physiological relations are all resolvable into mere resemblance; because every species is essentially distinct and separate from every other species; otherwise it would not be a species, but a variety.
The most similar species, therefore, are only allied to each other in consequence of the close resemblance of their general organisation; the degree of affinity being greater or less, according to the extent of that resemblance (according to the degree of their physiological, not their mere apparent, similitude); in short, according as they are more or less framed upon the same general or typical plan; which plans not only regulate the minutiae of structure in those species which are organised upon them, but, to a very considerable extent, even their colours and markings.
Of course, the observation here very naturally suggests itself, that, if the colours and markings of species have a definite use (which, in some instances, is sufficiently obvious even to our comprehension), then, we might reasonably expect to find that resemblance which is found to subsist between those of species whose habits are almost the same.
True; but, then, there are many trivialities observable in the marking of allied species, which can only be explained upon the principle that they are modifications of some particular general or typical plan, of markings, as well as of structure. Such is the pale line along the head of the newly discovered Dalmatian, Regulus modestus Gould, in place of the bright-coloured coronal feathers of its different congeners; which is exactly analogous to the curious fact, that the apparent rudiments of dentition exist in the gums of the foetal toothless whales; sufficiently intimating that these latter animals are modifications merely of some general typical plan, of which one of the leading characters is to be furnished with teeth.
So, also, might be adduced the tiny, soft, deflected spine situate at the bend of the wing of the common gallinule, in like manner indicating that this species, also, is framed upon some particular plan of structure, the more characteristic examples of which have spurred wings, as we find to be the case in the allied genus Parra.
In all the species organised upon any given type, we may always look for some trivial resemblances of this kind; we may always expect to find some traces of any particular structure or markings, which are observable in those typical forms of which the others are but modifications; the probability of this, of course, increasing with the number of degrees of affinity; and it is not unusual, too, to find colours or markings, which, in typical forms are scarcely discernible, developed, as it were, in particular modifications of those forms, to a considerable extent: yet, in the most approximate modifications of diverse subtypes of one general type, we only find such trivial resemblances of this kind as may be directly traced up to the typical standard from which they both diverge; whatever other marks of similitude these may show being obviously analogous adaptations, rather, to similarities of habit, unaccompanied by those trivial resemblances which imply physiological proximity.
Thus, however closely, both in form and colouring, our common grey flycatcher (Muscicapa grisola) may approximate to some of the smaller Tyrannulae of North America, the mottled character of its nestling garb at once indicates that it is not framed on exactly the same series of successive types; in a word, that its relation towards these tyrannules must be considered as one of approximation, rather than of direct affinity. It would be easy, in like manner, to illustrate the preceding several positions; but the limits of the present disquisition will not permit of it.
It only now remains for me to apply the various facts which I have been endeavouring to establish; after which I shall commence a minute detail of observations on the moulting of birds.
That our systems of classification should be founded on the true affinities of species, rather than upon any arbitrary characters, is now, I believe, admitted on all hands to be the desideratum; and the true principle on which alone this can be effected is, as it appears to me, sufficiently obvious; though, from our present very imperfect acquaintance with existing species, it must necessarily be a long while yet before our arrangements can be considered at all final, if, indeed, we can ever hope them to assume that character.
The true physiological system is evidently one of irregular and indefinite radiation, and of reiterate divergence and ramification from a varying number of successively subordinate typical plans; often modified in the extremes, till the general aspect has become entirely changed, but still retaining, to the very ultimate limits, certain fixed and constant distinctive characters, by which the true affinities of species may be always known; the modifications of each successive type being always in direct relation to particular localities, or to peculiar modes of procuring sustenance; in short, to the particular circumstances under which a species was appointed to exist in the locality which it indigenously inhabits, where alone its presence forms part of the grand system of the universe, and tends to preserve the balance of organic being, and, removed whence (as is somewhere well remarked by Mudie), a plant or animal is little else than a "disjointed fragment."
Systematists, with few exceptions, err most grossly in imagining that allied species have been created in direct reference to each other (as members of a sort of cabinet system of even proportions) rather than to the localities they indigenously frequent, to the office each was ordained to fulfil in the universal, or adaptive, system.
One would have supposed that the various facts which geology has brought to light would have sufficed to undeceive them in this particular. It cannot be too often repeated, that, upon whatever plan a species may be organised, its true relation (the reason for its existence at all) is solely connected with its indigenous locality: else, why should so many thousand species have ceased to be, the particular circumstances under which they were appointed to live no longer requiring their presence?
To expect, indeed, for a single moment, that, in any isolated class or division of organisms, a perfect system of another kind could obtain, harmonising in all points, and true in the detail to any particular number, appears to me (even Supposing that none of the species were now extinct, and that we knew all that are at present existing), prima facie, a manifest illusion. Species are distributed over the earth, wherever the most scanty means of subsistence for them are to be found; and their organisation is always beautifully and wonderfully adapted for obtaining it under whatever circumstances it may exist: just, therefore, as the surface varies, so do its productions and its inhabitants; and there is no locality, or apparently, even vegetable production, so peculiar, but species are found upon it especially organised to find their subsistence chiefly or wholly there.
The very underground lake has its own peculiar inhabitants; for the wondrous Proteus there revels in regions of everlasting night: of course happy in its existence as the bird that cleaves the free air, or as the lion that exults in his conquering prowess. Ponder this well; and it is clear, that upon these grounds alone all quinary imaginings must at once fall to the ground.
3. The "mineral kingdom" is a superfluous epithet, too vague to have any meaning beyond a negative one.
Chemically speaking, it, indeed, comprises both the others. The proper distinction is, of course,
between organised and not organised.