It indeed appears that, in these very typical genera, there is a sort of clustering of species (if I may be allowed the phrase) about the centre of radiation. In Ardea, and Corvus particularly, the central species become extremely difficult of determination; if, indeed, in some instances, the proximity is not even too close for detection.
   How nearly do some of the typical crows resemble! Upon the most scrupulous and minute comparison, C. L. Bonaparte was unable to discern the least difference between specimens of the European Corvus corone and the common crow of America; and he consequently infers their identity: yet who that attentively peruses the various descriptions of the latter, that considers well its marked gregarious habits, and the diversity of its note from that of our crow, can for a moment coincide with him in opinion?  Look again, to the raven, that formerly was considered a bird of universal distribution, as was also the snipe.
   First, the African species must be detached, as Le Vaillant's description of it should long ago have indicated; for we find that, independently of the differences in the bill, and certain particulars connected with its plumage, the proportionate size of the sexes is the reverse of that of the European species, as Le Vaillant himself ascertained and published.  Then the beautifully glossed raven of the Brazils is obviously distinct; also the raven of the South Sea Islands, and, there is reason to believe, that of India.  And what if these can be proved to be separate species, by fixed and constant structural distinctions; do they not show how nearly species may resemble, and point to the almost inevitable conclusion, that, in some instances, there may possibly be no means whatever of knowing them apart?

How vastly important is this consideration when we contemplate the natural productions of America!  Many years have now elapsed since the genius of Buffon suggested the capital proposition, that there is no absolute specific identity between any organism of the Eastern and Western continents, with the exception of those which inhabit very far to the north.  All subsequent investigation has gone to prove the force and acumen of this sterling remark; and the number of species (exclusive of evident stragglers) supposed to be common to the two continents has been gradually diminishing, on more careful and exact comparison from that time to the present.  In fact, I think we may now fairly venture to assume, putting aside stragglers, that those species alone are satisfactorily identical in the two continents which are distributed over the whole north of Asia, and may be looked for on the north-western coast of America.  Very lately, the American scaup (pochard) was found, on comparison, to be distinct from that of Europe, although the difference almost wholly consists in the obliquity of its wing spectrum; a character which, however, proved to be fixed and constant.  Had there not been this diversity, the two species would have been, of course, equally distinct: yet how should we have discriminated them apart?  The barn owls of the two continents, which are now believed (and on good grounds) to be distinct, are even more similar.⁸

Equally close resemblances obtain in other departments of the zoology of Europe and North America, and particularly in the insect tribes: many butterflies, for instance (as several of the Coliades), from the opposite shores of the Atlantic, being only to be told apart by the slowly acquired practical ken of the entomologist.  The natural productions of Japan, again, in many instances, present the most astonishing similitude to those of Europe; yet they exhibit characters which cannot be well reconciled with variation, however unimportant in themselves, because they are distinctions which climate or locality are not in the least likely to bring about.  Besides, supposing the latter, we should not only expect to meet with specimens in every degree intermediate, but to find the same species equally flexible to circumstances in other places, which is not the case.

In ornithology, the jay and bullfinch of Japan may be selected from among numerous other instances; the former differing only from the European bird in the greater development of certain markings about the head, and the latter presenting no other difference than the much paler, or roseate, tint of its abdominal plumage. Taking a series of species, we have every grade of diversity, from the obviously distinct Japanese peafowl (Pavo muticus), to the mealy linnet, which apparently, differs in no respect from that of Europe.  In a specimen of a pettychaps from the same locality, the only difference I could perceive from our common Sylvia trochilus on very minute inspection, consisted in a peculiar slight curve at the extremity of the upper mandible: still we know how nearly two British species of this genius resemble, and yet how very diverse are their notes.  Perhaps the song of the Japanese pettychaps is dissimilar from that of either: at any rate, a dry skin is hardly sufficient on which to found a definite opinion.

Of course, all these various facts lead us to the important consideration of, What is a species?  What constitutes specific distinction?  To which the only rational reply appears to be (and even this is quite incapable of probation), Beings derived from a separate origin.  For it appears that hybridism, after all, is but an uncertain guide, however satisfactory in particular cases; there being much reason to conclude, from a general survey of the facts recorded, that, as the degree of fertility in hybrids (paired with individuals of pure blood) varies according to the degree of proximity in the parent species, the possibility of mules being produced at all existing only within the sphere of a certain affinity; so, on the other hand, when the parent species approach so nearly as some that I have had occasion to mention, their mixed offspring would be almost equally prolific, hybrid with hybrid.  This is, at least, stated of all the members of the genus Bos; and most naturalists concur in the opinion, that our common fowls are derived from the blending of a plurality of species.
   Certainly, if the analogy of plants can be admitted, the fact is in so far settled; for I know many hybrid plants which of themselves yield fertile seed in abundance: the mixed produce, for example, of the Calceolaria purpurea and C plantaginea; the former a half-shrubby species, the latter herbaceous.⁹  A variety of additional instances could be enumerated.  Hybrid plants, however, are equally sterile with mule animals, if the parent species are not very closely allied.

It is to be hoped that, ere long, the experiments of the Zoological Society will have solved this curious and important problem.  Already some highly interesting and complex hybrids have been obtained under their management.¹⁰

I have found it to be a very general opinion among naturalists, that specific diversity must of necessity be accompanied by some perceptible difference in the structure.  To this I cannot accede, until I hear of a sufficient reason why it should be the case.  We perceive every grade of approximation, till in the shrews, for instance, a slight diversity in the form of one of the back teeth comprehends all the difference.  It is therefore presumed that, as so very trivial a deviation cannot be said to affect the animal's habits, for what purpose, then, does it exist, save to intimate the separateness of the species?  But, surely, it will not be contended that species were created with a view that man should be able to distinguish them!  Surely, differences w ere not imposed merely to facilitate the progress of human knowledge!
   Is it not much more rational to conclude, that, as great differences in the structure import corresponding diversities in habit, so, by the same rule, minor differences also imply an equivalent diversity in degree?  Let us, again, consider the American and European crows: here it would seem that specific diversity is unaccompanied by any structural deviation.

Of course, it is hardly necessary to hint the importance of these facts to geological enquirers: they intimate the excessive caution requisite ere we can venture to identify the fragments of an organism, when even existing species, in many instances, are not, probably, to be told apart. It must be admitted that they warrant a good deal of scepticism as to many of the identifications that have been assumed.

But to return now to the four typical genera, which have led to the above lengthy digression. I certainly do not conceive it necessary that there should be, in all instances, an unbroken gradation into the subordinate forms, similar to that from Merula into Philomela; for it is evident that the affinities of Philomela, and its relations to the thrush genus, would be the same, were there no intervening examples.  Still it is reasonable to suppose that, generally speaking, such series would occur; not, however, for the mere abstract purposes of arrangement, but because there are grades in localities and modes of life.
   That there should be species variously modified upon any particular plan of structure, and that the deviation should be greater in one instance than in another, of course implies radiation from a general centre; and the very circumstance that the same characters are more developed in one species than in another, necessarily also occasions a gradation in the particular direction, which may happen to be more or less regular, according as circumstances (adaptive relations) require.  That there should be a slight break, for instance, in the series where the fringillidous type is modified into Loxia, is perfectly consistent with the nature of the deviation; but the true affinities of the crossbills are, nevertheless, equally recognisable, and the same may be said in other cases where the hiatus is much more considerable.

And here it will not be out of place to say a few words upon the terms perfection, degradation, and the like, as applied to natural productions.  Let it be borne in mind, that, although every species is equally and wondrously perfect, even to the most trivial minutiae, in reference to the office for which it was designed, still, if we desire to cite an instance wherein the adaptation, if not more perfect, is, at least, more obviously remarkable and extraordinary than in another, it is to aberrant species, rather than to the central or typical exemplifications of a general plan of structure, that we must direct attention; inasmuch as the former exhibit those modifications of that plan, those adaptations to a peculiar mode of life, which are the most calculated to excite our wonder and admiration.
   Such forms as Loxia and Recurvirostra are sufficient illustrations of the position.  There, perhaps, would be no objection to the word degradation, understood strictly in a classical sense; but, when we consider its popular, its English meaning, in which alone it will be apprehended by an extensive class of readers, no term should be more carefully avoided: the most degraded species absolutely happening to be those which are the most worthy our especial admiration.

The difficulties of classification arise from the necessary fact (obviously necessary when we consider the adaptive relations of species) of there being successive centres of radiation; the different modifications of a leading plan of structure radiating in their turn, and thereby constituting an irregular series of subordinate types, of every degree of value. Thus, the starling type is comprehended in the omnivorous or corvine plan of structure, and, in its turn, comprises others of less importance, upon all of which may be organised an indefinite number of species, diversely modified to suit a variety of localities, and often approximating in external appearance to species framed upon other general types of structure, wherever they are alike modified to perform the same office in the adaptive system: such approximation, however, by no means inducing an additional degree of physiological affinity.

Before concluding this, I must call attention to another point worthy of consideration.  To recur again to the four typical genera we have all along been considering, and which, of course, it is most satisfactory to revert to in every instance, it appears that the central species, for the most part, exhibit a marked increase of size, being generally about the largest of those framed on their respective plans of structure.  I do not say that this obtains in every instance, but still it is so general as to be quite worthy of attention; and the rationale of it appears simply to be, that, as typical forms are more adapted for general distribution, and better calculated for finding subsistence in a variety of localities, than those modifications of them which are organised expressly for peculiar places only, we must infer that an increase of stature would, as a general rule, be incompatible with the well-doing of aberrant races; or, to put it inversely, that beings of comparatively large size require to be less partial in their adaptations; that (their wants being greater) they should not be too much confined to particular places for the needful supply of food.  However, this is a rule so broken into by exceptions, and so entirely dependent on the character of the particular adaptation, that, though obvious enough in the main, it is much more likely to meet with assent than demonstration.  Certain it is, that, in very many groups, the largest species are among the most centrally typical.  Witness, by way of example, the woodpeckers and the parrots.

In fabricating an arrangement according to the natural method (i.e., based on the true affinities of species), we cannot be too much impressed with the consideration that organisms must be ever regarded in their totality; that no one structural character can be expected to hold in all instances, however important in particular cases.  We have only to consider the fact, that, in a natural group, it is but the same leading plan of structure which is so variously modified, each organ, in its turn, being adapted differently to diverse circumstances; and we perceive how valueless are the arbitrary characters of those who try to frame artificial systems.
   Even the dentition of the Mammalia, so paramount in the majority of cases, becomes quite a secondary means of distinction in the Marsupialia; and the structure of the bill in birds, so important and corresponding a character in most instances, yet loses almost all its value in the Certhiadae.
Unquestionably, all the yoke-footed tribes are very nearly related by affinity; yet how discordant are they in the details of their structure!  A single, and comparatively trivial, resemblance in the organisation of the foot becomes, in this instance, a character of the very first importance.