Russel Wallace : Alfred Russell Wallace (sic)
Characters in the Classification
of Birds (S88: 1864)
This extract, I think, proves that I both fully appreciated the unity of this group and accurately defined its limits some years before M. Blanchard's publication; for though it is (in its separate form) altogether without date, yet he quotes work in 1857--a year after the publication of my paper.1
No one can be more convinced than myself of the utility of osteology, and especially of the sternum, in the classification of birds, and I sincerely trust this great work may be brought to a conclusion. I cannot, however, allow that osteological characters are an all-sufficing guide. Like every other character taken singly, osteology is a very uncertain and irregular test of affinity, and is, moreover, in almost every case accompanied by parallel external characters. Sometimes one, sometimes another part of the bird's organization has varied more rapidly, so that one group exhibits the most striking constancy of a part which in another group is subject to extreme modifications. The sternum is no exception to this rule, and by following it alone we should make the greatest errors in classification. For example, the sterna of the Finches and the Flycatchers are scarcely distinguishable, notwithstanding the great dissimilarity in almost every part of the structure of these birds--their bills, their feet, their plumage, their habits, food, and digestive organs. On the other hand, the sterna of the several genera of the Caprimulgidæ, differ from each other more than do those of the most distinct families of the restricted Passeres. The Bee-eaters, the Barbets, and the Woodpeckers, again, are three very distinct families, which, in a classification founded upon all parts of a bird's organization, cannot be brought in close contact; and yet their sterna, according to M. Blanchard, much resemble each other. It is evident, therefore, that the whole structure of a bird and its corresponding habits may be profoundly modified, and yet the sternum may retain a very close resemblance to a common form; and, on the other hand, the sternum may undergo important changes, while the general organization and habits are but little altered.
[[p. 39]] To prove that true affinities indicated by the sternum are also in most cases exhibited in external characters, it is only necessary to refer to the paper above quoted, in which the relation of the Hummers to the Swifts, and the separation of the Hornbills, the Rollers, the Musophagidæ, and the Parrots from the Passeres, were pointed out from the consideration of such characters alone. In that paper, however, I made two important errors, namely, putting the Todies with the Passeres (from the descriptions given of their habits), and including the Swallows among the Swifts. The character of the sternum is undoubtedly of great importance in finally setting such points as these.
I also at that time included the Psittaci among the Scansores; but I am now quite convinced that they deserve to rank as a primary division of the class of Birds, a rank to which the great peculiarity of the sternum, the large brain-cavity, and highly organized cranium fully entitle them.
With regard to M. Blanchard's determination of affinities from the body of the sternum only, without its appendages, I must remark that it often leads to erroneous results. For example, he says that the sterna of Merops and Tamatia do not differ enough to deserve a separate description; and he includes Megalæma with Tamatia in one section, as having the same form of sternum. He notices some differences in the Pìcidæ, but remarks on their resemblance to Megalæma and to the Toucans. Now in all these points an examination of the entire sternum, with the furcula, coracoids, and clavicles attached, leads me to very different results. The sterna of Merops and Nyctiornis, compared with those of two species of Megalæma, seem to me to show no resemblance whatever: in almost every part they present important differences of form, surface-texture, and proportions, while the furcula and coracoids are so different in the two, that I should unhesitatingly place them far apart, in at least different tribes or primary divisions of the Passeres. On the other hand, the sternum of the Toucans (Pteroglossus) resembles that of Megalæma most closely in every particular, and especially in the extreme weakness and complete separation of the two arms of the furcula--a character which I am not aware exists in any other families of birds. The sternum of the Picidæ [[p. 40]] presents many important differences from those of all these families, and fully bears out the isolation which their external characters exhibit. It differs much from Megalæma and Pteroglossus in its general form, as well as in details of structure, and still more from Merops. It seems to approach the typical Passeres more than either of the other groups to which M. Blanchard compares it; but its peculiar pyramidal shape, so remarkably narrowed at the anterior extremity, and its very short clavicles distinctly separate it as a characteristic and isolated form. It will, therefore, I think, be admitted that the affinities indicated by the complete sternum and appendages are much more in accordance with those derived from external form and structure, and from habits and economy, than those which M. Blanchard deduces from the body of the sternum alone.
These remarks are made in no spirit of depreciation of this very interesting and valuable work, but for the purpose of showing that isolated characters may lead to erroneous conclusions from whatever part of the organism they are chosen, and that in this respect osteological have no positive superiority over external characters. M. Blanchard tells us, in the introduction to this first instalment of his work, that he proposes to examine successively each separate part of the bird's skeleton. His future researches may therefore seriously modify the conclusions he has hitherto arrived at. I cannot but think, however, that he would have produced a more satisfactory work, if he had based it upon the comparison of the entire sternum, with its appendages attached, and also on the cranium, these two parts being of the greatest importance in classification.
It has been well observed by Professor Owen that those parts of an animal which have the latest immediate connexion with its habits and economy are exactly those which best exhibit deep-seated and obscure affinities. The wings, the feet, and the beak in birds may undergo the most extraordinary modifications in the same group in accordance with differences of habits and of external conditions, while at the same time such apparently insignificant characters as the general colouring, the texture of the plumage, the scaling of the tarsi, or the colour and texture of the eggs remain constant, and reveal the true relations of [[p. 41]] the species. Thus it is that the form of the sternum is of such importance, since it has no immediate dependence on external form and habits. The Sparrow, the Flycatcher, the Wren, and the Sunbird, all have one characteristic form of sternum; while between those of the Swallow and the Swift there is the greatest diversity.
It is evident also that the modifications of form immediately dependent on habits and external conditions are generally to be seen in the skin even better than in the skeleton of a bird. These are principally changes of form, size, and proportion in the bill, the feet, and the wings, which are excellent characters for distinguishing genera and even families; while for determining the true affinities of isolated groups we must have recourse to those characters which, having no direct dependence on habits, &c., are often persistent in a remarkable degree. Of these, no doubt, the sternum is of the highest value; but there are many others of almost equal importance. Such are the texture of the plumage; the form of the feathers and their arrangement over the surface of the body; the form of the nostrils; the scutellation of the tarsi; the mode of nidification, with the form, texture, and colour of the eggs; the covering of the young bird, and its changes of plumage; peculiarities of food, characteristic habits, and peculiar attitudes and actions.
As an instance of the value of such apparently trifling characters as the last, I may mention that the first time I saw a Roller (Coracias temmincki) alive, I was at once satisfied it was a Fissirostral bird, from a peculiar jerking motion of the head and tail when it alighted, which is common to Kingfishers, Trogons, Bee-eaters, and Motmots, but never seen in the typical Passeres. In like manner the motions of the Eurylæmi convinced me that they were not Fissirostres, but typical Passeres, as mentioned in my paper quoted at the beginning of these remarks (Ann. Nat. Hist. 1856, p. 199).
Now that true principles of classification are becoming so much better understood, we may, I think, hope that the chaos which has so long existed in ornithology will soon give way to a truly natural system which must obtain general acceptance.