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AMPHIBOLÆ, a group of birds so called by Nitzsch in 1829 (Observationes de Avium Carotide communi, p. 16) comprising the genera, as then understood, Musophaga (TOURACO), Colius (MOUSEBIRD), and Opisthocomus (HOACTZIN); but by no means to be confounded with the
AMPHIBOLI, one of Illiger's groups, defined in 1811 (Prodromus Systematis Mammalium et Avium, p. 203), and composed of the genera Crotophaga, Scythrops, Bucco, Cuculus and Centropus—the third of which is treated of under the titles of BARBET and PUFFBIRD, while the rest will be found under those of ANI, CHANNELBILL, and CUCKOW.
AMPHIBOLIC is a toe which can be reversed at will either backwards or forwards. The outer or fourth toe is amphibolic, and can be turned backwards in Pandion, the Striges, Musophagidæ, Leptosomatidæ, and Coliidæ. This feature, when retained, forms the true zygodactyle foot. . The MOUSE-BIRDS can turn the first toe forwards, being thus enabled temporarily to assume the condition of some of the SWIFTS, or that of zygodactyle birds. Reversion of the second toe backwards has produced the pseudo-zygodactyle or heterodactyle foot of the TROGONS (see SKELETON).
AMPHIMORPHÆ, the name given by Prof. Huxley (Proc. Zool. Soc. 1867, p. 460) to his second group of DESMOGNATHÆ, which consists of the genus Phoenicopterus (FLAMINGO), as being “So completely intermediate between the Anserine birds on the one side, and the Storks and Herons on the other, that it can be ranged with neither of these groups, but must stand as the type of a division by itself.”
ANATOMY (ůvatouía, dissection) is that branch of zoology which deals with the description of the organic structure of animals; a branch of this zootomy is Histology, the knowledge of the composition of the tissues of the various organs. The object of Comparative Anatomy is the explanation of the features exhibited by the animal organization. The comparative method examines numbers of different animals (or plants) with reference to the anatomical structure of their various organs, putting similar conditions together, and separating or excluding those which are dissimilar. By observing in such organs their size, number, shape, structure, relative position to other organs, and their development, we ultimately acquire a knowledge of such a series of conditions or features, exhibited by one and the same organ, which in their extremes may appear totally different, but are connected with each other by numerous intermediate stages. By proceeding in such a way, we are, for instance, enabled to understand the ankle-joint of Birds, by comparing the bones of their hind limbs with those of Mammals and Reptiles, and by concluding that the avine ankle-joint is produced by the fusion of the proximal tarsal bones with the tibia, and of the distal tarsals with the metatarsals, that consequently this joint in Birds is not the same as the ankle-joint of Mammals. If moreover, as is the case here, the study of the embryonic development of Birds shews that this fusion actually does take place, Ontogeny corroborates the correctness of the conclusions which we had arrived at by the strictly comparative or phylogenetic method.
Phylogeny, then, is the study of the relationship and the descent of the various animals, often with the help of fossil species, which are generally in some ways intermediate between other recent forms. For instance, through comparison of the skeleton of Birds with that of other Vertebrates, we find that Birds resemble Reptiles much more than they do Fishes or Amphibia or Mammals; this we express by saying that Birds are rather nearly related to Reptiles; the extraordinary resemblance of recent Birds with the fossil Archæopteryx, which at the same time has still many truly Reptilian characters, links the two classes still more together. We conclude that Reptiles and Birds are descendants of one common Reptilian stock. Since most Reptiles possess teeth, and the more than half avine Archæopteryx also has teeth, we again conclude that the earliest Birds likewise possessed such organs, and that their descendants have lost them. In this belief we are not shaken, although the most careful examination of embryonic birds has failed to reveal even the smallest traces of dental germs. The subsequent discovery in American cretaceous deposits of Toothed birds, like Enaliornis and Hesperornis, is a beautiful corroboration of the soundness of the method.
Ontogeny, on the other hand, includes the study of the development of the individual, and hence is often called EMBRYOLOGY. Whatever organic modifications the parents have acquired during their life, subjected to the struggle for existence, be it through natural or sexual selection, or be it through spontaneous variation, will be inherited, at least partly, by their offspring. Ontogeny is therefore the recapitulation by the growing individual of the sum total of the ever-changing stages and conditions through which the whole chain of its ancestors has passed : it is a condensed repetition of Phylogeny. This repetition is often so much condensed that many previous stages are rapidly passed through, or may even be apparently left out, or they have become modified beyond recognition through the development of organs necessitated by, and restricted to, the embryonic stages. Such strictly embryonic organs (for instance the AMNION and the ALLANTOIS, or the placenta) are features which have originally nothing whatever to do with the adult, because we know of no Vertebrates which in their adult condition live within such bags. Another imperfection of the ontogenetic record lies in the fact that the sequence in which the various organs are developed in the embryo does not always correspond with the temporary succession in which we know them to have been acquired during the phylogenetic development of the animal in question ; thus feathers begin to bud while the skeleton of the embryo is still cartilaginous. Such discrepancies between the ontogenetic and phylogenetic development have been termed “cænogenetic” by Prof. Hæckel (from kaivos, new). The fact of their frequent occurrence without our being aware of the various cases, warns us to be extremely careful in interpreting the various features exhibited by the embryo. In the present state of our knowledge it is often impossible to decide the taxonomic value of a given feature.
Descriptive Anatomy requires a number of technical terms which shall not be ambiguous, or permit of doubt as to their intended meaning. For instance, terms like upper and lower, anterior and posterior, inner and outer, are often liable to be misunderstood. In ordinary parlance anterior corresponds with ventral in Man (with reference to whom many of our technical terms have been invented), but the head though at the anterior end of the animal is not ventral, and yet the anterior surface of a vertebra may mean its ventral surface. In fact, these vernacular names change their meaning according to the starting point which happens to be used.
It seems therefore advisable to enumerate, and give a definition of, those terms which it is useful to apply throughout in the description of the various organs of a Bird.
The longitudinal axis of every bird corresponds with its vertebral column: one end is marked by the head, the other by the tail, thus giving the terms cephalic and caudal ; and concerning the neck, trunk, and tail, together with their constituent parts, anterior and posterior. On one side of the vertebral column or axis are situated the heart, lungs, and digestive organs ; this is the ventral, in opposition to the dorsul side. These give, combined with anterior and posterior, right and left. An axis at right angles with the longitudinal one, and at the same time running right and left, is a transverse axis ; beginning with the vertebral axis as the starting-point, the terms proximal and distal are applied to any organ or part which is referable to the longitudinal axis. These two terms are chiefly applicable to parts like ribs and limbs with their various elements. The proximal end of the tibia articulates with the distal end of the femur; the proximal end of a rib articulates with a vertebra, and so on. The tip of the wing marks its distal, the AXILLA its proximal end.
With reference to an ideal plane through the longitudinal axis, and at right angles to the transverse axis, are applied the terms median or inner, lateral or outer. Lastly, since it is not always obvious to which axis or plane a given organ is to be referred, its parts can be described with reference to its neighbours. Hence we speak of the tibial and fibular, radial and ulnar side of the bones and other parts of the extremities; the fourth toe is on the fibular, outer, or lateral side of the foot, the first, which is ordinarily the hind toe, on the tibial, inner, and posterior side.
The basal part of an organ is generally also its proximal part or root, while the apex corresponds with its free or distal end, the latter being the portion most removed or distant from the region whence it grew. Thus we speak of the distal tracheal rings as joining the bronchi, while proximally the trachea is attached to the larynx.
In comparing the various parts of one animal with each other, or with those of another animal, we call the organs which are morphologically or structurally similar homologous, the parts which physiologically or functionally correspond are analogous. When the comparison is restricted to one individual, the homologies are general. The different vertebræ, or the ribs, or the anterior and posterior extremities of any particular Bird are serially homologous or homodynamous organs, because they are to a certain extent repetitions of each other, although not necessarily exactly alike. If the comparison refers to similar organs in various individuals, no matter if these belong to the same species, genus, family, or class, the homologies are special, and these again may be complete or incomplete. For instance, the humerus of a Bird is completely homologous with that of a Mammal, Reptile, or Amphibian ; the atlas or first vertebra of a Crow is completely homologous with the same part of a Dog. On the other hand, the wing of a Crow is only incompletely homologous with the arm of Man; nor is the two-toed foot of the Ostrich completely homologous with the four-toed foot of a Fowl, although the various bones which compose the feet in both are complete homologues.
Homologous organs are consequently developed from the same parts of the embryos of the creatures which are under comparison. Hence the number of existing homologies in given animals indicates their further or closer relationship, and is used for assigning these animals to their places in the system. It follows from this consideration, that the animal's place in the system depends greatly, or entirely, upon the characters or organs selected for this purpose. Unless all the organs and all their characters are carefully considered, not only in the few Birds which happen to occupy our attention at the time, but also in Birds of as many different groups as it is possible to examine, our attempts to produce a classification of Birds must invariably end in the production of arbitrary “keys." It is extremely difficult, often hopeless, with the present state of our knowledge of the anatomy of Birds, to decide which characters and which organs are of extrinsic taxonomic value, and which are not. Nor is it always possible to see why certain organs, fully developed, and exhibiting striking and constant features in one group of Birds, are extremely variable in another otherwise very circumscribed and apparently natural group. Supposing such a character to be absent in a given group, is it absent because it has not yet been developed, or is it because it has been lost? Has it been lost by the ancestors of this group, or has it been abolished within this group ? In the former case the absence of this character would probably help to decide the relative position of the group; in the latter case this very same character would be reduced to a diagnostic point within the group, and not throw any light upon its relationship or systematic position. It may be very easy to diagnose genera or even large groups of birds, but this ability to determine them by the help of mechanically arranged “keys” does not necessarily afford us more than an occasional glimpse of the sunk avine tree, at the reconstruction of which we all aim, as the true representation of the natural affinities of Birds.
It is occasionally insisted upon that “tact” will help us to select and to reject characters, and thus prevent us from falling into glaring errors; but tact is a personal feeling, often bias, and it is proof, not inclination, that settles scientific questions. The importance of these considerations, often expressed before in abler words, is gaining more and more ground among ornithologists, and will therefore permit the following illustrations of the ways in which we may or may not apply the study of comparative anatomy to classification.
The presence of the AMBIENS Muscle is a Reptilian feature; among Birds it exists in the majority of the lower groups, and is absent in most of the higher members of the Class. We conclude that the latter have lost this muscle, and not that it has not yet been developed in them. Its reduction or loss is still going on within some groups, such as Parrots and Pigeons. This loss takes place independently in widely different groups. It follows, first, that absence of this muscle does not always indicate relationship; secondly, that we can derive forms that are without it from a group which still possess it ; but that the reversed conclusion is not possible. We know of no organ which has been redeveloped after it has once disappeared in the ancestors of the animals under consideration. Therefore the absence of the ambiens muscle in all Owls, which apparently use their hinder extremities in the same way as the Falconida (which possess this muscle), indicates that the Owls are not developed from the Falconida, but from a group which, like the Macrochires, had already lost this organ.
Similar arguments apply to the CÆCA. It is generally admitted