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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 dorsal 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.
Homologous Organs
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 vertebrae, 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 Orow 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 on 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.
Illustration: Presence of the Ambiens and Caeca
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 Falconidae (which
possess this muscle), indicates that the Owls are not developed from the
Falconidae, but from a group which, like the Macrochires, had
already lost this organ.
Similar arguments apply to the caeca. It is generally
admitted that the ancestral bird-stock did possess well-developed caeca,
therefore all those birds which are now found without caeca must have lost
them either phylogenetically or even during their embryonic development.
In fact, we find in embryos of such birds as have, when adult, only very
small or rudimentary caeca, that the germs of these organs are, in the
embryo, just as well developed as in birds with long caeca; but these organs,
in a Pigeon for instance, do not grow any further. They are in early life
stopped in their development, and thus remain in a rudimentary state. Again,
in all those birds which are completely devoid of caeca, their suppression
is simply carried out to the extreme. We cannot therefore, as has been
done sometimes, separate Birds into those with and those without caeca:
this is especially wrong, as there exist many forms, which, although undoubtedly
allied to each other, differ greatly in the presence or absence of these
organs. If we want to use the caeca as a differentiating character, we
must consider their quality, and enquire whether those organs are functional
and well developed, or are they now without function? Consequently
birds with rudimentary caeca have to be grouped together with those which
have no caeca, although the ancestors of both had functional caeca; and
since we know that these organs stand in close correlation with the nature
of the food, we are enabled to weigh their taxonomic value. Hence it is
probable that the Owls are related to the caeca-possessing Nightjars, and
that the caecaless Macrochires (like Swifts)
are a recent offshoot of the latter, while it is impossible to assume that
the Owls are descendants of the Diurnal Birds-of-Prey.
The modifications of the Carotid Arteries enabled
Professor Fuerbringer to draw a very ingenious and valid conclusion as
to the probable original centre of the Parrots. While the Australian, Oriental,
and African Parrots exhibit almost every possible modification of these
arteries, from the most primitive to the most specialised conditions, the
American Parrots possess only the right deep carotis and a left superficial
carotis, an arrangement which is a decidedly recent, not primary feature.
Hence the conclusion that the American Parrots are a branch of the Palaeotropical
stem; but however fascinating such speculations are, we must not forget
that they hardly ever amount to definite proofs.
Supposing we divide Birds into two classes (A and
B), according to the presence or absence of the Ambiens muscle. As a second
differentiating character let us take the functional or fully developed
(a) and the absent or functionless state of the CAECA (b); and as
a third character the presenc (Ð) or absence (ß)
of an Aftershaft. Then using the ambiens as the principal, and the
aftershaft as the tertiary differentiating feature, and indicating presence
or absence by the signs + and - respectively, we get the following eight
divisions:-
A. Ambiens +
a. Caeca +
Ð. Aftershaft +
e.g. Gallinae, Impennes, Pheonicopterus, Musophaga, etc.
ß. Aftershaft
- e.g.
Anseres, etc.
b. Caeca -
Ð. Aftershaft +
e.g. Accipitres, Psittaci partim.
ß. Aftershaft
- e.g.
Columbae partim.
B. Ambiens -
a. Caeca +
Ð. Aftershaft +
e.g. Alca, Podicipes.
ß. Aftershaft
- e.g. Striges.
b. Caeca -
Ð. Aftershaft +
e.g. Psittaci pt., Cypseli, Trochili, etc.
ß. Aftershaft
- e.g. Passeres, Columbae pt., Herodii, etc.
Thus the Owls in this arrangement approach nearest
to the Auks and Grebes, while the Parrots, owing to their variable ambiens
muscle, are grouped either with the Accipitres, or with the Swifts and
Humming-birds. This is obviously unsatisfactory, perhaps owing to the value
of the ambiens muscle being overrated. Let us next use the aftershaft as
the principal, the ambiens as the secondary determining character, and
the caeca as the third. Then the Psittaci approach the Gallinaceous birds
and also the Auks and Grebes, while the Owls verge into the neighbourhood
of Pigeons, Herons, and Passerine birds. Again, by using the caeca as the
principal, and the ambiens as the secondary feature, Psittaci, Accipitres,
and Columbre, Owls, Auks, and Grebes are once more thrown together. The
same or very similar arrangements result from a combination of the caeca
with the oil-gland, or of the ambiens and caeca with the conditions of
the palatal bones. But these persistent coincidences will never induce
us to look upon them as indicating relationship between Owls, Auks, and
Grebes, because this conclusion would be obviously wrong! How does the
question stand with regard to other combinations, when we cannot at a glance
discern a glaring error? When, e.g. according to the muscles
of the thigh, leaving out the ambiens, Striges, Accipitres, and Cypselidae
stand closely together? Is this a mere coincidence or does a deeper
meaning underlie this Trias? It is obviously not due to a superior taxonomic
value of Garrod's myological formulae, because application of the same
principle throws Nightjars, Storks, and Parrots together.
Conclusion
It is hopeless to attempt to arrive at a natural
classification of Birds by a mechanical arrangement of even a great number
of alleged leading characters. More may be expected from the combination
of various taxonomic arrangements, each of which has been based upon a
single organic system without reference to other organs. Of course everyone
of such one-sided attempts will occasionally show a rather perplexing face,
but each of them will bring to light some unexpected points of resemblance
between certain groups; and, while restricting ourselves to one organic
system, we are more likely to understand which points are given to modifications
through mode of life, food, habit, and surroundings, and which remain least
affected, and therefore are indicative of relationship. Let us then combine
the several one-sided arrangements. They will each of them contribute something
good or certain, and thus help to settle the great question. Reasoning
from a broad basis of facts will do the rest.
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