Numerous as are the writers in this department of zoology; assiduously as the study of birds is cultivated in all parts of the civilised world; and talented as are many of the naturalists and close observers who devote their more particular attention to this branch; it still appears to me, that the numerous and very diversified regular changes of plumage and general external appearance, observable in this interesting subclass of animals, have been hitherto very greatly and strangely overlooked, and that, in consequence, the many valuable physiological inferences deducible from their investigation have been quite lost to the purposes of science and of classification.
   It is true that many naturalists have in so far attended to the mutations of plumage which some particular species undergo, as that they are able at once to recognise them in every livery they assume; but the exact ages, and seasons, of moulting; the precise nature of the general, or only partial, change that is undergone, and the various accordances and dissimilarities observable between the changes of distinct species; the endless characters of agreement and difference, so important in pointing out affinities, in showing what apparently similar races could never be brought to hybridise together; would seem to have been passed over as unworthy of notice, as undeserving of a particular investigation.

The subject is both extensive and complicated, and involves a number of other recondite inquiries.  I could have wished that some naturalist better qualified than myself had taken it in hand.
   For my own part, I have little time for practical observation; but, having long been in the habit of keeping a number of birds (chiefly the smaller kinds which occur in Britain) in a state of captivity, I have thus enjoyed some very favourable opportunities for making myself fully acquainted with the various changes that a great number of species undergo, both seasonally, and in their progress from youth to maturity and old age; and I have neglected no opportunity of studying those of other races, which circumstances may have variously chanced to place in my way.

It is to be remarked, then, that some species of birds (as, for example, the larks and starlings, the crows, the woodpeckers, and various others) moult the whole of their immature, or nestling, plumage the first year, including the wing and tail primaries; while a very few (as the bearded pinnock, Calamophilus biarmicus, and rose muffin [Parus caudatus Linnaeus], Mecistura rosea) shed the primary feathers of the tail the first season, but not those of the wing: numerous other races (as all the modifications of the fringillidous and thrush types) moult their clothing plumage very soon after leaving the nest, and retain the primaries till the second autumn; the Falconidae, again, and some others, undergo no change whatever until that period.
   All those which I have as yet mentioned change their feathers only once in the year, towards the close of summer, immediately on the cessation of the duties towards their progeny: but there are various other tribes (as the wagtails and pipits, Motacillinae, and most of the aquatic races) which regularly undergo another general moulting in the spring; though in no instance, that I am aware of, are the primary wing feathers shed more than once in the year: those of the tail, however, in some rare instances, are; and the different coverts, together with the secondary and tertiary wing feathers, in most, if not all, double-moulting birds, are changed twice.
   In some migrative species (as the cuckoo, and most of the swallows), the young of the year do not change their plumage until the winter months; whereas the old birds moult in autumn; and in other birds, again (as in various ducks, [VIII, 544, 545]), two general changes of feather take place within the short period of about four months.  Very many other similar diversities, of a more or less subordinate character, might be enumerated, if enough have not been already mentioned to show that a wide field for observation is here open to the practical ornithologist.

In like manner may analogous diversities be observed throughout the mammiferous subclass of vertebrate animals; thus, the squirrels and the shrews renew their covering twice in the year, and the rats and rabbits but once.  The common squirrel's seasonal changes have never, that I am aware of, been remarked by any naturalist, though it is so common an inhabitant of our island: its summer coat is very different from that of winter, the fur being much coarser, more shining, and of a bright rufous colour; while the ornamental tufts to the ears are wholly wanting: these grow in autumn, while the animal is renovating its coat, and continue usually till about the beginning of July, the time varying somewhat in different individuals.
   Their winter fur, besides being of a much finer quality and texture, is considerably longer, thicker and more glossy, and quite of a different hue from that of summer, inclining to greyish brown.  The first young ones, too, which are produced very early in the season, push forth the winter garb, which, I believe, they then retain throughout the summer; whereas the second race of young ones, which, for the most part, make their appearance about midsummer, are first clad in the summer dress, which is exchanged, before they have become half grown, for that of winter.  It is not improbable, also, that diversities of a like kind may obtain in the renewal of the scales of fishes.

What the definite purpose effected by very many of these peculiar and dissimilar changes may be, I confess myself utterly unable to say; nor can I suggest even a plausible hypothesis upon the subject.  Why, for example, should the pipits (Anthus) shed their plumage twice in the year, and the larks (Alauda) but once?  And why, also, should the latter change all their nestling primaries at the first moult, while the former retain theirs until the third (including the vernal) general renovation of plumage?
   It is easy enough to say, with Mr. Mudie, that, in the wagtails, and certain other species, the colours of the summer and winter dresses are each, in so far as they differ, more peculiarly adapted to the particular season of the year; but this is merely a concomitancy. in other words, this adaptation is not the purpose of the change; for we find that, in certain species which regularly moult twice in the year (as the tree pipit), the summer and winter plumage hardly differ; whilst, on the other hand, as complete an adaptation of colour to season is effected in others (as the stone chat, and most of the Fringillidae), which moult in autumn only, by the wearing off of the extreme tips of the feathers; these in winter having covered and concealed another, and, in many instances, a very diverse, colour beneath.

By what reason can we ever hope to account for the curious fact, that the common drake, and also the pintailed and other teals, should moult their whole clothing plumage (including the tail) in summer, and then again in autumn?  As Mr. Waterton has well remarked on the subject, "All speculation on the part of the ornithologist is utterly confounded; for there is not the smallest clue afforded him, by which he might be enabled to trace out the cause of the strange phenomenon.
   To Him alone, who has ordained the Ostrich to remain on the earth, and allowed the bat to soar through the ethereal vault of heaven, is known why the drake, for a very short period of the year, should be so completely clothed in the raiment of the female, that it requires a very keen and penetrating eye to distinguish them." [VIII, 544.]  In one point of view , however, at least, a knowledge of these changes is of considerable practical use to the naturalist; for they not unfrequently point out at once, in doubtful cases, the most appropriate situation of a genus in a system, and thus assist him very greatly in his endeavours to fabricate a sound system of classification.  Instances of this I shall not here advance, as it is necessary to say something first of what meaning I attach to that most hackneyed of all phrases, "natural system," concerning which it is more than probable that my views may very considerably, and perhaps essentially, differ, from those of many who may perchance honour them with a perusal.

Under this phrase, then, two very distinct kinds of relation are ordinarily blended together and confounded; viz., the adaptive relation of every organised production to the conditions under which it was appointed to exist, and the physiological relation subsisting between different species of more or less similar organisation.  These may be aptly designated the adaptive system, and the physiological system; the system of relative adaptation between the earth, its productions, and its inhabitants, and the system of agreements and differences between the organisation of distinct races.
   To illustrate the former of these is, perhaps, superfluous: it is the system by which alone the existence of one species is necessary to that of another, and which binds each race to its locality; where the presence of each is alike necessary to preserve the equilibrium of organic being around; and when circumstances have changed, and the necessity for its agency no longer remains, a whole race perishes, and the fragments of a skeleton in the solid rock perhaps alone proclaim that such had ever existed.  It is the grand and beautiful, the sublime and comprehensive, system which pervades the universe, of which the sun and planets are but a portion, and which, to return to ornithology, is so well exemplified in the adaptation of the ptarmigan to the mountain top, and the mountain top to the habits of the ptarmigan; which suits the ostrich to the arid desert, the woodpecker to the forest, and the petrel to "the far sea wave."  It is the majestic and admirable system by which all nature works so beautifully together, and to which all that our external senses reveal appertains.  It is the system which, exquisite and intensely interesting in all its minutest details, is, if possible, even more so in its complicated relations; by which, by the unity of design pervading which, all is demonstrable to be the workmanship of One omnipotent and all-foreseeing Providence, under the beneficent dispensation of whom nought that ever exists or occurs stands isolated and alone, but all conduce and work admirably together for the benefit of the whole; by whose all-wise decree it is ordained, that, while the lofty and sterile mountain peak attracts the clouds, which in winter, in consequence, precipitate themselves upon it in the form of snow, it should thus cause itself to become clad in the hue of all others the most calculated to prevent its internal temperature from being farther reduced, and itself from thereby becoming an increased source of cold by radiation to all around; while, at the same time, the concretion of snow itself, instead of deluging the country round with superfluous moisture, is thus retained for a time upon the heights, not only to shelter the more tender organised productions of the mountain from severer cold, but also to furnish, by the action of the summer sun, a due supply of water, when needed, to the fountains and rills which irrigate and fertilise the more level country. there having done its part, to flow on to the mighty reservoirs of the ocean, again to arise in clouds, and to fulfill again its appointed rounds, with perpetual never ceasing energy, while the world endures.

Look around our world,. behold the chain of love
Combining all below and all above.
See plastic Nature working to this end;
The single atoms each to other tend,
Attract, attracted to, the next in place
Form 'd and impell'd its neighbour to embrace.
See matter next, with various life endued,
Press to one centre still, the general good.
See dying vegetables life sustain,
See life dissolving vegetate again:
All forms that perish other forms supply:
(By turns we catch the vital breath and die,)
Like bubbles on the sea of matter borne,
They rise, they break, and to that sea return.
Nothing is foreign: parts relate to whole;
One all-extending all-preserving Soul
Connects each being, greatest with the least;
Made beast in aid of man, and man of beast;¹
All served, all serving; nothing stands alone:
The chain holds on, and, where it ends, unknown.
POPE'S Essay on Man, EPISTLE III

Every species of organism, as must be obvious to all examining thinking persons, is framed upon a greater or smaller series of successively subordinate typical plans, upon each of which is organised a variety of different species, perfectly unconnected and distinct from each other, however some may resemble, even to minutiae, and which exhibit each typical or subtypical structure more or less modified, and, in the extremes, generally more or less approximating towards the extreme modifications of other plans of organisation, in direct relation to the endless diversifications of the surface of the earth, to variety of climate, or to peculiar modes of procuring sustenance.

Thus far, I believe, all systematists agree.
I must venture, however, to differ from the majority of them, in opposing the prevalent notion, that the extreme modifications of diverse types blend and inosculate² by direct affinity; contending that, however closely these may apparently resemble, the most similar modifications of diverse types are not, in a physiological sense, more nearly related to each other than are the more characteristic examples of the same.

To this conclusion I was originally led by reflection on various interesting phenomena connected with the changes of plumage which take place in birds; having observed that, however importantly, to suit peculiarity in the mode of life, the general structure of very aberrant forms may be modified, so as to render it even doubtful upon which fundamental type they are organised there are, notwithstanding, certain constant characters, of less importance to the existence and welfare of the species, by which every typical standard may be easily traced to its ultimate ramifications; some of the most valuable of these characters, in the feathered race, being afforded by peculiarities in the mode of moulting.  To illustrate this, I may cursorily adduce the various finchlike Sturnidae (Aglaius, Molothrus, Dolichonyx, etc.); extreme modifications of the Corvus type; as are also, however unlike they unquestionably appear, the genera Alauda, and even Ammodramus.  All these, I have ascertained either from direct observation, or from competent sources, shed the nestling primaries the first season, which is not the case with any modification of the fringillidous type, or of the dentirostral.

If other characters be wanting, which point alike to the same conclusion, I may mention the constant presence of a craw, or enlargement of the oesophagus, in all the Fringillidae, and its invariable absence in all, even the most aberrant, modifications of the Corvus type; all the latter, too, preserve the ambulatory mode of progression, which, in perfection, is not observable in any Fringillidae, not even Plectrophanes.  Again, other characters of distinction between these two equivalent divisions are sufficiently visible in the general aspect of the bill, even where the extremes approximate: all the Fringillidae, for instance (to which I would restrict the appellation Conirostres), possess what may be strictly defined a bruising, or compressing, instrument: whereas the general character of the same organ in the other division is rather what may be aptly termed a thrusting one, intermediate in its structure between those of the Fringillidae and Dentirostres; in which last group the bill is modified into either a snapping, holding, or tugging instrument, as the case may be: sometimes all three, as in vireo.

However, to return to the proposition I was just advancing, that, physiologically speaking, there are no combinations of distinct types, no intermediate organisms, save those between a central type and its ultimate ramifications: the general structure may be intermediate, and, consequently, the situation a species holds in the adaptive system, the office which it may have to perform in the general economy of the universe; but the latter does not constitute affinity; neither, strictly speaking, is it analogy; therefore I must distinguish it by another term, approximation.

As I shall have occasion to make use of these words frequently, as I proceed, it will be necessary, before advancing further, to define the precise meaning which I attach to them, however much this may appear digressing from the subject more immediately in hand.