Chapter XXI: Part 21

LITERATURE.--Well nigh complete lists of the enormous myological
literature are contained in Fuerbringer's _Untersuchungen zur
Morphologie und Systematik der Voegel_, and in Gadow's vol. _Voegel_ of
Bronn's _Klassen und Ordnungen des Tierreichs_. Only a few papers and
works can be mentioned here, with the remark that few authors have
paid attention to the all-important innervation of the muscles. A.
Carlsson, _Beitraege zur Kenntniss der Anatomieder Schiwmmvoegel_; K.
Svensk, _Vet. Ak. Handlinger. J.G._ No. 3 (1884); A. Alix, _Essai sur
l'appareil locomoteur des oiseaux_ (Paris, 1874); H. Gadow, _Zur
vergl. Anat. der Muskulatur des Beckens und der hinteren Gliedmasse
der Ratiten_,4 deg. (Jena, 1880); A.H. Garrod, "On Certain Muscles of the
Thigh of Birds and on their value in Classification," _P.Z.S._, 1873,
pp. 624-644; 1874, pp. 111-123. Other papers by Garrod, 1875,
pp. 339-348 (deep planter tendons); 1876, pp. 506-519 (wing-muscles of
Passeres), &c.; J.G. de Man, _Vergelijkende myologische en
neurologische Studien over Amphibien en Voegels_ (Leiden, 1873),
(_Corvidae_); A. Milne-Edwards, _Recherches anatomiques et
paleontologiques pour servir a l'histoire des oiseaux fossiles de la
France_ (Paris, 1867-1868), tom. i. pls. ix.-x. (_Aquila_ and
_Gallus_); R. Owen, article "Aves," Todds' _Cydopaed. of Anat. and
Phys._ i. (London, 1835); "On the Anatomy of the Southern _Apteryx_,"
_Trans. Zool. Soc._, iii., 1849; A. Quennerstedt, "Studier i foglarnas
anatomi," _Lunds Univers. Aarsk._, ix., 1872 (hind-limb of swimming
birds); G. Rolleston, "On the Homologies of Certain Muscles connected
with the Shoulder-joint," _Trans. Linn. Soc._, xxvi., 1868; R.W.
Shufeldt, _The Myology of the Raven_ (London, 1891); M. Watson,
"Report on the Anatomy of the _Spheniscidae_," _Challenger Reports_,
1883.

3. _Nervous System._

_Brain._--The more characteristic features of the bird's brain show clearly a further development of the reptilian type, not always terminal features in a direct line, but rather side-departures, sometimes even a secondary sinking to a lower level, and in almost every case in a direction away from those fundamentally reptilian lines which have led to the characters typical of, and peculiar to, the mammals.

The forebrain forms the bulk of the whole brain, but the large size of
the hemispheres is due to the greater development of the basal and
lateral portions (_pedunculi cerebri_ and _corpora striata_), while
the pallium (the portion external to the lateral ventricles) is thin,
and restricted to the median side of each hemisphere. As a direct
result of this undoubtedly secondary reduction of the pallium--due to
the excessive preponderance of the basal and lateral parts--the corpus
callosum (i.e. the transverse commissure of the right and left
pallium) is in birds reduced to a narrow flat bundle of a few white
fibres; it is situated immediately above and behind the much stronger
anterior commissure, i.e. the connexion between the corpora striata,
or chief remaining part of the hemispheres. Owing to the small size of
the olfactory lobes the anterior arms of the latter commissure are
wanting. There is very little grey matter in the cortex of the
hemispheres, the surface of which is devoid of convolutions, mostly
quite smooth; in others, for instance pigeons, fowls and birds of
prey, a very slight furrow might be compared with the Sylvian fissure.

The Thalamencephalon is much reduced. The epiphysis, or pineal body,
is quite as degenerate as in mammals, although still forming a long
stalk as in reptiles. In birds, this stalk consists entirely of
blood-vessels, which in the adult enclose no terminal vesicle, and
fuse with the membranous linings of the skull. The midbrain is
represented chiefly by the optic lobes, the cortex of which alone is
homologous with the _corpora quadragemina_ of the mammals. Their
transverse dorsal connexion is the posterior commissure; otherwise the
whole roof portion of the midbrain is reduced to a thin membrane,
continuous with that which covers the Sylvian aqueduct, and this
ventricle sends a lateral cavity into each optic lobe, as is the case
in reptiles. The right and left lobes themselves are rent asunder (so
to speak), so that they are freely visible from above, filling the
corners formed by the hemispheres and the cerebellum. The latter is,
in comparison with mammals; represented by its middle portion only,
the _vermis_; in a sagittal section it shows an extremely well
developed _arbor vitae_, produced by the transverse, repeated folding
of the whole organ. In comparison with reptiles the cerebellum of
birds shows high development. Forwards it covers, and has driven
asunder, the optic lobes; backwards it hides the much shortened
medulla oblongata.

Several futile attempts have been made to draw conclusions as to the
intelligence of various birds, from comparison of the weight of the
whole brain with that of the body, or the weight of the hemispheres
with that of other parts of the central nervous system.

The _brachial plexus_ is formed by four or five of the lowest cervical
nerves; the last nerve of this plexus often marks the boundary of the
cervical and thoracic vertebrae. The composition of the plexus varies
much, not only in different species, but even individually. The most
careful observations are those by Fuerbringer. The serial number of
these nerves depends chiefly upon the length of the neck, the extremes
being represented by _Cypselus_ (10th-14th cervical) and _Cygnus_
(22nd-24th), the usual numbers of the common fowl being the 13th-17th
nerves.

The _Crural Plexus_ is divided into a crural, ischiadic and pubic
portion. The first is generally composed of three nerves, the hindmost
of which, the _furcalis_, issues in most birds between the last two
lumbo-sacral vertebrae, and then divides, one half going to the
crural, the other to the sciatic portions. The _obturatorius_ nerve
invariably comes from the two main stems of the crural. The ischiadic
portion consists generally of five or six nerves, which leave the
pelvis as one thick system through the ilio-ischiadic foramen. The
last nerve which contributes to the ischiadic plexus leaves the spinal
column in most birds either between the two primary sacral vertebrae,
or just below the hindmost of them, and sends a branch to the pubic
portion which is composed of post-ischiadic nerves, partly imbedded in
the kidneys, and innervates the ventral muscles between the tail and
pubis, together with those of the cloaca and copulatory organs.

The _Sympathetic System_ forms a chain on either side of the vertebral
column. In the region of the neck lateral strands pass through the
transverse canal of the cervical vertebrae; but from the thoracic
region onwards, where the cardiac branch to the heart is given off,
each strand is double and the basal ganglia are successively connected
with the next by a branch which runs ventrally over the capitulum of
the rib, and by another which passes directly through the foramen or
space formed between capitulum and tuberculum. In the pelvic region,
from about the level of the posterior end of the ischiadic plexus, the
strand of each side becomes single again, passing ventrally over the
transverse processes. Lastly, towards the caudal region the right and
left strands approach and anastomose, eventually coalescing in the mid
line.

LITERATURE.--A. Bumm, "Das Grosshirn der Voegel," _Zeitschr. wiss.
Zool._, 38, 1883, pp. 430-466, pls. 24-25; F. Leuret and P. Gratiolet,
_Anatomie comparee du systeme nerveux_ (Paris, 1839-1857), with atlas;
A. Meckel, "Anatomie des Gehirns der Voegel," in _Meckel's Archiv f.
Physiol._ vol. ii.; H.F. Osborn, "The Origin of the Corpus Callosum, a
contribution upon the Cerebral Commissures of the Vertebrata,"
_Morphol. Jahrbuch_, 1886, xii. pp. 223-251, pls. 13-14; M.A.
Schulgin, "Lobi optici der Voegel," _Zool. Anzeig._ iv. pp. 277 and
303; E.R.A. Serres, _Anatomie comparee du cerveau_ (Paris, 1824, 4
pls.); L. Stieda, "Studien uber das centrale Nervensystem der Voegel
und Saeugethiere," _Zeitschr. wiss. Zool._ xix., 1869, pp. 1-92, pls.;
J. Swan, _Illustrations of the Comparative Anatomy of the Nervous
System_ (London, 1835, 4to, with plates).

Concerning the spinal nerves and their plexus: H. v. Jhering, _Das
peripherische Nervensystem der Wirbeltiere_ (Leipzig, 1871); W.A.
Haswell, "Notes on the Anatomy of Birds," _Proc. Linn. Soc. N.S.W._
iii., 1879; M. Fuerbringer, "Zur Lehre von den Umbildungen der
Nervenplexus," _Morph. Jahrb._ v., 1879, p. 358.

4. _Organs of Sense._

The _Eye_ is essentially reptilian, but in sharpness of vision, power and quickness of accommodation it surpasses that of the mammals. The eyeball, instead of being globular, resembles rather the tube of a short and thick opera-glass.

The anterior half of the sclerotic is composed of a ring of some ten
to seventeen cartilaginous or bony scales which partly overlap each
other. Another cartilage or ossification, the posterior sclerotic
ring, occurs within the walls of the posterior portion of the cup, and
surrounds, especially in the Pici and in the Passeres, the entrance of
the optic nerve. The iris is in most young birds at first brown or
dull-coloured, but with maturity attains often very bright tints which
add considerably to the charm of the bird; sexual dimorphism is in
this respect of common occurrence. The iris contains a sphincter and a
dilator muscle; the former, supplied by branches from the
_oculomotorius_ nerve, is under control of the will, whilst the
dilator fibres belong to the sympathetic system. When fully dilated,
the pupil is round in all birds; when contracted it is usually round,
rarely oval as in the fowl. From near the entrance of the optic nerve,
through the original choroidal fissure, arises the much-folded pecten,
deeply pigmented and very vascular, far into the vitreous humour. The
number of its folds varies considerably, from three in _Caprimulgus_
to nearly thirty in crow (_Corvus_). _Apteryx_, which since Owen has
generally been stated to be devoid of such an organ, likewise
possesses a pecten; its base is, however, trumpet-shaped, covers
almost the whole of the optic disk, and extends nearly to the lens in
the shape of a thick, densely pigmented cone, without any plications,
resembling in these respects the pecten of many Lacertilia (see G.L.
Johnson, _Phil. Trans._, 1901, p. 54). In the retina the cones prevail
in numbers over the rods as in the mammals, and their tips contain, as
in other Sauropsida, coloured drops of oil, mostly red or yellow. Near
the posterior pole of the fundus, but somewhat excentrically placed
towards the temporal or outer side, is the _fovea centralis_, a slight
depression in the retina, composed almost entirely of cones, the spot
of most acute vision. Many birds possess besides this temporal fovea a
second fovea nearer the nasal side. It is supposed that the latter
serves monocular, the other the binocular vision, most birds being
able to converge their eyes upon one spot. Consequently the whole
field of vision of these birds possesses three points where vision is
most acute. It may here be remembered that of the mammalia man and
monkeys alone are capable of convergence, and have a circumscribed
macular area.

Of the outer eyelids, the lower alone is movable in most birds, as in
reptiles, and it frequently contains a rather large saucer-shaped
cartilage, the _tarsus palpebralis_. The margins of the lids are
sometimes furnished with eyelashes, e.g. in the ostrich and in the
Amazon parrots, which are vestigial feathers without barbs. During the
embryonic stage the lids are fused together, and either become
separated shortly before the bird is hatched, as is the case with most
Nidifugae, or else the blind condition prevails for some time, in the
young Nidicolae. All birds have, like most reptiles, a well-developed
third lid or "nictitating membrane," which moves from the inner
canthus obliquely upwards and backwards over the cornea. The moving
mechanism is a further and much higher development of that which
prevails in reptiles, there being two muscles completely separate from
each other. Both are supplied by the _abducens_ nerve, together with
the _rectus externus_ muscle. One, the _quadratus_ or _bursalis_
muscle, arises from the hinder surface of the eyeball, and forms with
its narrow margin, which is directed towards the optic nerve, a pulley
for the long tendon of the _pyramidalis_ muscle. This arises from the
nasal surface of the ball, and its tendon passes into the somewhat
imperfectly transparent nictitating membrane. The quadrate muscle
adjusts the motion, and prevents pressure upon the optic nerve; during
the state of relaxation of both muscles the nictitans withdraws
through its own elasticity.

See R. Leuckart in Graefe and Saemisch's _Handbuch d. Ophthalmologie_
(Leipzig, 1876, vol. i. chap. 7); H. Mueller, _Gesammelte Schriften_
(Otto Becker, Leipzig, 1872), and _Arch. f. Ophthalmol._ iii.; Ch.
Rouget, "Recherches anatomiques et physiologiques sur les appareils
erectiles," "Appareil de l'adaptation de l'oeil" ... _Compt. Rend._
(Paris, xlii., 1856, pp. 937-941); M. Schultze, art. "Retina," in
Stricker's _Handbuch der Gewebelehre_, 1871, vol. ii.; J.R. Slonaker,
"Comp. Study of the Area of Acute Vision in Vertebrates," _Journ.
Morph._, 1897.

_Ear._--The outer opening of the ear is, with rare exceptions, concealed by feathers, which are often rather stiff, or modified into bristles. There is no other protection, but slight, imperfectly movable folds of skin arise from the outer rim. The largest ear-opening is met with in the owls, with correspondingly larger folds of skin, the function of which is less that of protection than, probably, the catching of sound. In many owls the right and left ears are asymmetrical, and this asymmetry affects the whole of the temporal region, all the bones which surround the outer and middle ear, notably the squamosal and the quadrate, so that the skull becomes lopsided, one ear being turned obliquely down, the other upwards. (For, detail see Collett, _Christiania Vidensk. Forhandl._, 1881, No. 3.)

The middle ear communicates with the mouth by the Eustachian tubes,
which pass between the basisphenoid and basioccipital bones, and unite
upon the ventral side of the sphenoid, a little behind its
articulation with the pterygoids, where they open into the mouth
cavity by a short membranous duct. The columellar apparatus, or
auditory chain of ossicles (fig. 16), extending between the fenestra
ovalis and the tympanic membrane or drum, consists of (1) the long and
slender columella, a straight, ossified rod which fits with a disk
into the fenestra ovalis; it is homologous with the stapes (_m.st._),
although not stirrup-shaped; (2) the extracolumellar mass. This is
chiefly cartilaginous and sends out three processes: the dorsal (_s.
st._) is attached to the upper wall of the drum cavity; the outermost
(_e. st._) is fastened on to the middle of the drum membrane; the
third, ventral or infracolumellar process (_i. st._) is directed
downwards and tapers out into a thin, partly cartilaginous, strand,
which originally extended to the inner corner of the articular portion
of the mandible, but on its long way comes to grief, being squeezed in
between the pterygoid and quadrate. This long downward process being
homologous with an almost exactly identical arrangement in the
crocodile, and with the _processus folii_ of the mammalian malleus, it
follows that the whole extracolumellar mass, that between stapes and
drum, is equivalent to incus and malleus of the mammalia. There is, in
birds, no _annulus tympanicus_. Birds possess an ear-muscle which at
least acts as a _tensor tympani_; it arises near the occipital
condyle, passes through a hole into the tympanic cavity, and its
tendon is, in various ways, attached to the inside of the membrane and
the neighbouring extracolumellar processes.

As regards the inner ear, the endolymphatic duct ends in a closed
_saccus_, imbedded in the dura mater of the cranial cavity. The apex
of the cochlea is turned towards, and almost reaches the anterior wall
of the occipital condyle; at most it makes but half a twist or turn;
it possesses both Reissner's membrane and the organ of Corti. Although
the _scala tympani_ is so rudimentary, not reaching a higher level
than in most of the reptiles, and remaining far below the mammalia,
birds do not only hear extremely well, but they distinguish between
and "understand" pitch, notes and melodies.

See G. Breschet, _Recherches anatomiques et physiologiques sur
l'organe de l'audition chez les oiseaux_ (Paris, 1836), with Atlas; C.
Hasse, various papers in _Zeitschr. f. wiss. Zool_. vol. xvii, and in
_Anatomische Studien_, pts. ii. and iv. (Bresku, 1871); I. Ibsen,
_Atlas anatomicus auris internae_ (Copenhagen, 1846); G. Retzius, _Das
Gehororgan der Wirbelthiere_ (Stockholm, 1884), ii. pp. 139-198, pls.
15-20.

_Nose._--The olfactory organ is poorly developed, and it is still a question whether birds possess much power of smell; many are certainly devoid of it.

The olfactory perceptive membrane is restricted to the posterior
innermost region of the nasal chamber, where it covers a slight
bulging-out prominence on the nasal wall. This so-called third, upper
or posterior conch is not a true conch, nor is that of the vestibulum;
only the middle one forms a scroll, and this corresponds to the only
one of reptiles and the lower of the mammals. The nasal cavity
communicates with the mouth by the choanae or posterior nares,
situated between the palatine process of the maxillary, the palatine
and the vomer. The outer nares or nostrils are most variable in size
and shape. In the Steganopodes they tend to become much reduced, e.g.
in cormorants (_Phalacrocoracidae_), and especially in _Sula_, where
the nasal slits become completely closed up, and the greater portion
of the nasal cavity is also abolished, being restricted to the
olfactory region with its unusually wide choanae. The nasal septum is
often more or less incomplete, producing _nares peniae_, e.g. in the
Cathartae, in the Anseres, gulls, rails and various other aquatic
birds. The secretions of the mucous membrane of the nasal cavity, and
a pair of naso-lacrymal glands (not to be confounded with the
Harderian and the lacrymal glands), moisten and clean the chamber. The
glands are variable in size and position; when very large, e.g. in
plovers, they extend upon the forehead, causing deep impressions on
the bones of the skull. Jacobson's organ has been lost by the birds,
apparently without a trace in the embryonic fowl, but T.J. Parker has
described vestiges of the corresponding cartilages in the _Apteryx_
(_Phil. Trans._, 1890).

See C. Gegenbaur, "Ueber die Nasenmuscheln der Voegel," _Jena Zeitschr_.
vii., 1873, pp. 1-21.

5. _Vascular System._

The _heart_ lies in the middle line of the body, its long axis being parallel with that of the trunk. The whole ventral surface of the pericardium is exposed when the sternum is removed. The right and left halves are completely divided by septa, no mixture of the venous and arterial blood being possible, an advance upon reptilian conditions, even the highest.

The atria are comparatively small, the walls being thin, especially
those of the right, which possesses numerous muscular ridges
projecting into the cavity presenting a honeycombed appearance. The
interauricular septum is mostly entirely membranous; in the middle it
is thinner, rather transparent, but there is no depression or _fossa
ovalis_. The whole sinus venosus has become part of the right atrium.
It receives the three great venous trunks of the body, namely the
_vena cava superior dextra_, the _vena cava superior sinistra_ more
dorsally, and the _vena cava inferior_ more to the right and below;
the opening of the last is guarded by two prominent valves in place of
the mammalian _valvula Eustachii_. The right ventricle occupies the
ventral portion of the heart. The communication with the atrium is
guarded by a _valvula cardiaca dextra_, which only in function
represents the mammalian tricuspid; it consists of an oblique
reduplication of the muscular fibres together with the endocardiac
lining of the right ventricle, while the opposite wall is convex and
forms neither a velum nor papillary muscles, nor _chordae tendineae_.
The right anterior corner of the right ventricle passes into the short
stem, guarded by three semi-lunar valves, which divides into the two
pulmonary arteries. There are likewise two pulmonary veins, entering
the left atrium by one orifice. Two or three membranous flaps, held by
numerous _chordae tendineae_, form a true mitral valve, and allow the
blood to pass through the left _ostium atrioventriculare_. The blood
leaves the heart past three semi-lunar valves, by the _right_ aorta,
this being alone functional, a feature characteristic of, and peculiar
to, birds. Remnants of the left aortic arch persist sometimes in the
shape of a ligamentous strand. The aortic trunk is very short, sends
off the coronary arteries and then the left _aorta brachiocephalica_,
while the rest divides into the right brachiocephalic and the _aorta
descendens_. Each brachiocephalic soon sends off its subclavian, while
in the normal or more usual cases the rest proceeds as the carotid
trunk, inclusive of the vertebral artery. But the carotids show
several interesting modifications which have been examined chiefly by
C.L. Nitzsch and by A.H. Garrod. (1) The right and left carotids
converge towards the middle and extend up the neck, imbedded in a
furrow along the ventral surface of the cervical vertebrae. This is
the usual arrangement. (2) The two carotids are fused into one
_carotis conjuncta_, imbedded in a special median osseous semicanal of
the vertebrae; e.g. herons, flamingos, and some parrots. (3) There is
one _carotis conjuncta_, but the basal portion of its original right
component is obliterated, leaving a so-called _c. primaria sinistra_,
an unfortunate name. Such _Aves laevocarotidinae_ of Garrod are
common, e.g. all the Passeriformes. (4) The reverse of the third
modification, producing a _c. primaria dextra_ in the bustard
_Eupodotis_. In other likewise very rare cases a left, or a left and
right, superficial carotids are developed and take the place of the
then vanished deep or primary carotids.

_Venous System._--The bird's liver receives nearly all the blood from
the stomach, gut, pancreas and spleen, as well as from the left liver
itself, into the right hepatic lobe, by a right and left portal vein.
The _venae hepaticae magnae_ join the _vena cava posterior_ and
thereby form with it the _vena cava inferior_. The left _hepatica
magna_ receives also the umbilical vein, which persists on the
visceral surface of the abdominal wall, often anastomosing with the
epigastric veins. A likewise unpaired vena _coccygeo-mesenterica_ is
usually present. There is no renal portal system, excepting
unimportant vestiges of such a system in the head kidneys.

_Lymphatic System._--The white blood-corpuscles are produced in the
follicles at the base of the intestinal villi. The lymph vessels of
the tail and hinder parts of the body enter the hypogastric veins; and
at the point of junction, on either side, lies a small lymph heart,
which often persists until maturity. The red blood-corpuscles are
invariably oval disks, with a central nucleus which causes a slight
swelling; hence they are oval and biconvex.

See A.H. Garrod, "On the Carotid Arteries of Birds," _Proc. Zool.
Soc._, 1873, pp. 457-472; E.A. Lauth, "Memoire sur les vaisseaux
lymphatiques des oiseaux," _Ann, Sci. nat._ (iii. 1824), p. 381; J.J.
Mackay, "The Development of the Branchial Arterial Arches in Birds,
with special reference to the Origin of the Subclavians and Carotids,"
_Phil. Trans._ 179 B (1888), pp. 111-141; L.A. Neugebauer, "Systema
venosum avium," _Nov. Act. Leopold. Carol._ xxi., 1844, pp. 517-698,
15 pls.; R. Gasch, "Beitraege zur vergl. Anatomic des Herzens der Voegel
und Reptilien," _Arch. f. Naturgesch._, 1888.

6. _Respiratory System._

The _lungs_ are small and occupy only the dorsal portion of the thoracic cavity. There is only one right and one left lobe, each traversed through its whole length by a _mesobronchium_, whence arise about ten secondary bronchia; these send off radially arranged _parabronchia_, which end blindly near the surface. The walls of these tertiary tubes send out, in all directions, _canaliculi aeriferi_ which, ending in slight swellings, recall the mammalian _aveoli_.

Highly specialized air-sacs are characteristic of all birds. They are very thin-walled membranes, very poor in blood-vessels, formed by the bulged-out pleural or peritoneal covering of the lungs, through the parabronchial tubes of which they are filled with air. Their function is not quite clear. The usual suggestion, that the warm air contained within them assists the bird in flight, balloon-like, is absurd. They assist in the extremely rapid and vigorous ventilation of the lungs, the latter being capable of but very limited expansion and contraction in birds. Exchange of gas through the walls of the air-sacs, almost devoid of blood-vessels, can at best be much restricted.

There are five pairs of larger sacs belonging to the pulmonary
system:--(1) prebronchial or cervical, extending sometimes far up the
neck, even into the cranial cavities; the throat-bags of the prairie
fowls (_Cupidonia_ and _Pedioecetes_) are a further development; (2)
subbronchial or interclavicular; (3 and 4) anterior and posterior
thoracic or intermediate; (5) abdominal sacs. Most of these extend
through narrow apertures--_foramina pneumatica_--into the hollow
bones, sometimes, e.g. in hornbills and screamers, into every part of
the skeleton, or, in the shape of innumerable pneumatic cells, even
beneath the skin. There is also a naso-pharyngeal or tympanic system
of air-sacs, restricted to the head (cf. the _siphonium_ described in
connexion with the mandible), but filling also such curious organs as
the frontal excrescence of _Chasmorhynchus_, the Brazilian bell-bird,
the throat-bag of the adjutant stork, and the gular pouch of the
bustard.

The _trachea_ or windpipe is strengthened by numerous cartilaginous,
often osseous, complete rings, but in the emeu several of these rings
are incomplete in the medioventral line, and permit the inner lining
of the trachea to bulge out into a large neck-pouch, which is used by
both sexes as a resounding bag. In humming-birds and petrels the
trachea is partly divided by a vertical, longitudinal, cartilaginous
septum. In some of those birds which have a peculiarly harsh or
trumpeting voice, the trachea is lengthened, forming loops which lie
subcutaneously (capercally, curassow), or it enters and dilates the
symphysis of the furcula (crested guineafowl); or, e.g. in the cranes
and in the hooper swan, even the whole crest of the sternum becomes
invaded by the much elongated, manifolded trachea.

The _syrinx_ or lower larynx is the most interesting and absolutely
avine modification, although absent as a voice-producing organ
(probably due to retrogression) in most Ratitae, storks, turkey
buzzards (_Cathartes_) and Steganopodes. The syrinx is a modification
of the lower part of the trachea and of the adjoining bronchi.
Essential are vibrating membranes between the cartilaginous framework,
and next, special muscles for regulating the tension. The majority of
birds possess a pair of internal tympaniform membranes forming the
inner or median walls of the bronchi, which are there furnished with
semi-rings only. External tympaniform membranes exist, with great
variations, between the specialized one or two last tracheal and some
of the first bronchial rings.

According to the position of the chief sound-producing membranes,
three types of syrinx are distinguishable:--(1) Tracheo-bronchial, by
far the commonest form, of which the two others are to a certain
extent modifications. The essential feature is that the proximal end
of the inner membranes is attached to the last pair of tracheal rings;
outer tympaniform membranes exist generally between the 2nd, 3rd and
4th bronchial semi-rings. This type attains its highest development in
the Oscines, but it occurs also in many other orders. (2) Syrinx
_bronchialis_. The outer membranes are spread out between two or more
successive bronchial semi-rings, a distance from the trachea which is,
in typical cases, devoid of sounding membranes; some Cuculi,
Caprimulgi, and some owls. (3) Syrinx _trachealis_. The lower portion
of the trachea consists of thin membranes, about half a dozen of the
rings being very thin or deficient. Inner and outer membranes may
exist on the bronchi. The _Tracheophonae_ among the Passeriformes, the
possessors of this specialized although low type of syrinx, form a
tolerably well-marked group, entirely neotropical. But indications of
such a syrinx occur also in _Pittidae_, pigeons and gallinaceous birds
(_Gallidae_), the last cases being clearly analogous.

Whilst the type of syrinx affords no help in classification, it is
very different with its muscles. These--as indicated by their supply
from a branch of the hypoglossal nerve, which descends on either side
of the trachea--are, so to speak, a detached, now mostly independent
colony of glosso-pharyngeal muscles. Omitting the paired
tracheo-clavicular muscles, we restrict ourselves to the syringeal
proper, those which extend between tracheal and bronchial rings. Their
numbers vary from one pair to seven, and they are inserted either upon
the middle portion of the bronchial semi-rings (_Mesomyodi_), or upon
the ends of these semi-rings where these pass into the inner
tympaniform membrane (_Acromyodi_). The former is morphologically the
more primitive condition, and is found in the overwhelming majority of
birds, including many Passeriformes. The acromyodian type is
restricted almost entirely to the Oscines. Further, according to these
muscles being inserted only upon the dorsal, or only upon the ventral,
or on both ends of the semi-rings, we distinguish between _an_-,
_kat_- and _diacromyodi_. But the distinction between such _Acromyodi_
and the _Mesomyodi_ is not always safe. For instance, the _Tyranninae_
are anacromyod, while the closely allied Pipras and Cotingas are
katacromyod; both these modifications can be shown to have been
derived but recently from the weak meso- and oligomyodian condition
which prevails in the majority of the so-called _Oligomyodi_. On the
other hand, the diacromyodian type can have been developed only from a
strong muscular basis which could split into a dorsal and a ventral
mass; moreover, no Passeres are known to be intermediate between those
that are diacromyodian and those that are not.

Attempts to derive the anacromyodian and the katacromyodian from the
diacromyodian condition are easy on paper, but quite hopeless when
hampered by the knowledge of anatomical facts and how to use them.
There remains but one logical way, namely, to distinguish as
follows:--(1) _Passeres anisomyodi_, in which the syrinx muscles are
unequally inserted, either on the middle or on one end of the
semi-rings, either dorsal or ventral. This type comprises the
Clamatores. (2) _Passeres diacromyodi_, in which some of the syrinx
muscles are attached to the dorsal, and some to the ventral ends,
those ends being, so to say, equally treated. This type comprises the
Oscines. Both types represent rather two divergent lines than
successive stages, although that of the Clamatores remains at a lower
level, possessing at the utmost three pairs of muscles, whilst these
range in the Oscines from rarely two or three to five or seven.

This way of using the characters of the syrinx for the classification
of the Passeriformes seems simple, but it took a long time to
accomplish. Joh. Mueller introduced the terms _Polymyodi_ and
_Tracheaphones_, Huxley that of _Oligomyodi_; Mueller himself had,
moreover, pointed out the more important characters of the mode of
insertion, but it was Garrod who invented the corresponding terms of
_Acro_-and _Mesomyodi_ (= _Tracheophones_ + _Oligomyodi_). (For
further historical detail, see ORNITHOLOGY). After W.A. Forbes had
investigated such important genera as _Philepitta_ and _Xenicus_, P.L.
Sclater, A. Newton and R.B. Sharpe divided the Passeres respectively
into _Oscines, Oligomyodae, Tracheophonae_ and _Pseudoscines_
(= _Suboscines_); _Oligomyodae, Tracheophonae_ and _Acromyodae;
Oscines, Oligomyodae, Tracheophonae_ and _Atrichiidae_. Ignoring the
fact that some _Oligomyodae_ are meso- and others acromyodian, they
tried to combine two irreconcilable principles, namely, mere numbers
against quality.

BIBLIOGRAPHY.--M. Baer, "Beitr. z. Kenntniss d. Atemwerkzeuge bei den
Voegeln," _Zeitschr. wiss. Zool_. lxi. 1896, pp. 420-498; Campana,
_Physiologie de la respiration chez les oiseaux. Anatomie de
l'appareil pneumatique_ ... (Paris, 1875); A.H. Garrod, "Major
Divisions of Passerine Birds (syrinx, &c.)," _P.Z.S._, 1876, pp.
506-519; and "On the Conformation of the Thoracic Extremity of the
Trachea in the Class _Aves_," _P.Z.S._, 1879, pp. 357-380; J. Mueller,
_Stimmorgane der Passerinen_, Mueller's Arch. (1847); and _Abh. Akad.
Wiss_. (Berlin, 1845-1847), translation by F.J. Bell, Oxford, 1878; H.
Strasser, "Luftsacke der Voegel," _Morph. Jahrb_. iii., 1877, pp.
179-227; C. Wunderlich, "Unterer Kehlkopf der Voegel," _Nov. Act. Leop.
Carol_., 1884; Ph. C. Sappey, _Recherches sur l'appareil respiratoire
des oiseaux_ (Paris, 1847); W.A. Forbes, "Contributions to the Anatomy
of Passerine Birds (syrinx)," _P.Z.S._, 1880, pp. 380-386, 387-391;
1881, pp. 435-737; 1882, pp. 544-546, 569-571; W. Yarrell,
"Observations on the tracheae of Birds," _Trans. Linn. Soc_., 1827,
pp. 378-391.

7. _Digestive System_.

For a general account of the digestive organs, see ALIMENTARY CANAL. Here only a few peculiar features may be mentioned.

The young pigeons are fed by both parents with a peculiar stuff, the
product of the strongly proliferating epithelial cells of the crop,
which cells undergo a cheese-like fatty degeneration, and mixed with
mucus, perhaps also with the proventricular juice, make up a milk-like
fluid. Should the young die or be removed during this period, the
parents are liable to die, suffering severely from the turgid
congestion of the hypertrophied walls of the crop.

The male of the hornbills, _Bucerotinae_, feeds his mate, which is
imprisoned, or walled-up in a hollow tree, during the whole time of
incubation, by regorging his food. This bolus is surrounded, as by a
bag, by the cast-up lining of the gizzard. Since this process is
repeated for many days the habitual reaction of the stomach well-nigh
exhausts the male. A graphic account of this is given in Livingstone's
travels.

The hoactzin, _Opisthocomus_, feeds to a great extent upon the leaves
of the aroid _Montrichardia_ or _Caladium arborescens_. The crop is
modified into a large and very rugose triturating apparatus, while the
gizzard, thereby relieved of its function, is reduced to the utmost.
The large and heavy crop has caused a unique modification of the
sternal apparatus. The keel is pushed back to the distal third of the
sternum, whilst the original anterior margin of the keel is
correspondingly elongated, and the furcula fused with the rostral
portion.

In the ostrich, _Struthio_, the craze of overloading the stomach with
pebbles which, when triturated into sand, are not voided, has brought
about a dislocation, so that the enormously widened and stretched
space between proventriculus and gizzard forms a bag, directed
downwards, whilst the gizzard itself with part of the duodenum is
rotated round its axis to more than 100 deg.. A similar rotation and
dislocation occurs in various petrels, in correlation with the
indigestible sepia-bills, &c., which these birds swallow in great
quantities. In _Plotus_, the snakebird, the pyloric chamber of the
stomach is beset with a mass of hair-like stiff filaments which permit
nothing but fluid to pass into the duodenum. The gizzard of various
birds which are addicted to eating hairy caterpillars, e.g. _Cuculus
canorus_ and trogons, is often lined with the broken-off hairs of
these caterpillars, which, penetrating the cuticle, assume a regular
spiral arrangement, due to the rotatory motion of the muscles of the
gizzard.

8. _Cloaca and Genital Organs._

The cloaca is divided by transverse circular folds, which project from
its inner walls, into three successive chambers. The innermost, the
coprodaeum, is an oval dilatation of the end of the rectum, and
attains its greatest size in those birds whose faeces are very fluid;
it serves entirely as the temporary receptacle of the faeces and the
urine. The next chamber, the urodaeum, is small, and receives in its
dorso-lateral wall the ureters and the genital ducts; above and below
this chamber is closed by circular folds, the lower of which, towards
the ventral side, passes into the coating of the copulatory organ when
such is present. The urodaeum serves only as a passage, the urine
being mixed with the faeces in the chamber above. The third or
outermost chamber, the proctodaeum, is closed externally by the
sphincter ani; the orifice is quite circular. It lodges the copulatory
organ, and on its dorsal wall lies the _bursa Fabricii_, an organ
peculiar to birds. It is most developed in the young of both sexes, is
of unknown function, and becomes more or less obliterated in the
adult. Only in the ostrich it remains throughout life, being
specialized into a large receptacle for the urine, an absolutely
unique arrangement. A true urinary bladder, i.e. a ventral dilatation
of the urodaeum, is absent in all birds. It is significant that the
whole type of their cloaca much resembles that of the Crocodilia and
Chelonia, in opposition to that of the Lacertilia.

The penis, and its much reduced vestige of the female, is developed
from the ventral wall of the proctodaeum. It occurs in two different
forms. In the Ratitae, except _Rhea_, it consists mainly of a right
and left united half (_corpora fibrosa_), with a deep longitudinal
furrow on the dorsal side, and much resembles the same organ in
crocodiles and tortoises. It is protruded and retracted by special
muscles which are partly attached to the ventral, distal end of the
ilium. Another type exists in _Rhea_ and in the _Anseriformes_,
greatly specialized by being spirally twisted and partly reversible
like the finger of a glove. This is mainly due to the greater
development of an unpaired, median portion, analogous to the mammalian
_corpus spongiosum_, which is much less prominent in the Ratitae; the
muscles of this type are derived solely from the anal sphincter. In
other Carinatae, e.g. tinamous and storks, the penis is very much
smaller and simpler, with every appearance of a degenerated organ. In
the great majority of birds it has disappeared completely and the
primitive way of everting the cloaca is resorted to.

Both right and left testes are functional. They become greatly
enlarged in the breeding season; in the sparrow, for instance, from
the size of a mustard seed to that of a small cherry. The vas deferens
descends with many undulations down the lateral side of the ureter of
the same side, and opens upon a small papilla into the urodaeum.
Extraordinary increase in length during the breeding season causes the
vasa deferentia in some of the African weaver-birds to protrude, or to
bulge out the cloacal walls beyond the vent. The spermatozoa exhibit
many differences in shape, size and proportions, in the various groups
of birds. They have been studied minutely by E. Ballowitz.

Only the left ovary becomes functional, with rare individual
exceptions. Both present the appearance of diminutive clusters of
grapes, at the anterior end of the kidneys, close to the suprarenal
bodies, separated from each other by the descending aorta and by the
vena cava where this is formed by the right and left _vena iliaca
communis_. During the breeding season many more eggs are developed
than reach maturity, amounting in most birds to several dozens. Those
germs which do not ripen during the season undergo a process of
resorption, and in the winter the whole ovary dwindles to often a
diminutive size. In young birds both oviducts are almost equal in
size, but the right soon degenerates into an insignificant strand.
During every laying season the left duct increases enormously by new
formation of its component fibres. For instance, in the fowl its
volume increases about fifty-fold, growing from some 6 in. in length
and scarcely one line in width to more than 2 ft. in length and 1/2 in.
in thickness. The upper, wide opening of the duct is attached by
elastic, peritoneal lamellae to the hinder margin of the left lung;
the middle portion of the duct is glandular and thick-walled, for the
deposition of the albumen; it is connected by a short, constricted
"isthmus" (where the shell-membrane is formed) with a dilated "uterus"
in which the egg receives its calcareous shell and eventual
pigmentation.

BIBLIOGRAPHY.--A. v. Brunn, _Rueckbildung nicht ausgestossener
Eierstockseier, Henle Festschrift_ (Bonn, 1882); E. Ballowitz, "Die
Spermatozoen der Voegel," _Arch. Mikr. Anat._ xxxii., 1888, pls. 14-18;
M. Sacchi, "Contribuzione all' istiologia del ovidotto dei
saurop-sidi," _Att. Soc. Ital._, Milano, vol. xxx.; W.A. Forbes, "On
the Bursa Fabricii in Birds," _P.Z.S._, 1877, pp. 304-318; H. Gadow,
"Remarks on the Cloaca and on the Copulatory Organs of the Amniota."
_Phil. Trans._, 1887, pp. 5-37, pls. 2-5; Martin Saint Ange, "Etude de
l'appareil reproducteur dans les cinq classes d'animaux vertebres,"
_Mem. Ac. Soc._, Paris, xiv., 1856; E. Retterer, "Contribution a
l'etude du cloaque et de la bourse de Fabricius," _Robin's Journ. de
l'anat. et physiol._, 1885, pp. 369-454, pls. 17-19.

B. FOSSIL BIRDS

Much had naturally been expected from the study of fossil birds, but, so far as the making of classifications is concerned, they have proved rather a source of perplexities. So long as the characters of new fossils are only of specific and generic value, it is mostly possible to assign the birds to their proper place, but when these characters indicate new families or orders, for instance Hesperornithes, Ichthyornithes, Palaelodi, their owners are put outside the more tersely constructed classifications applicable to modern birds. It is no exaggeration to say that the genus, often even the species, can be determined from almost any recent bone, but in the case of Miocene, and still more, of Eocene fossils, we have often to deal with strange families, which either represent an extinct side branch, or which connect several recent groups with each other. Our artificially-established classifications collapse whilst we gain further insight into the mutual affinities of the existing groups. Of course this must be so if evolution is true. But it also follows that, if every extinct and recent bird were known, neither species, nor genera, nor families, nor orders could be defined. We should be able to construct the pedigree of every group, in other words, the gigantic natural system, but there would be no classification. Much light has also been thrown by fossil birds upon the study of geographical distribution. The key to the distribution of recent groups lies in that of the extinct forms. Not only have many absolutely new families been discovered, but many kinds of modern birds are now known to have existed also in countries which they are now extinct. There were, for instance, trogons, secretary-birds, parrots, and other now Ethiopian forms in Miocene France. Ostriches, undistinguishable from _Struthio_, have been found in Samos and in the Sivalik Hills.

The proper study of fossil birds may be said to have begun with A. Milne-Edwards, whose magnificent _Oiseaux fossiles de la France_ was published from 1867 to 1871. This work deals chiefly with mid-Tertiary forms. A new impetus was given by O.C. Marsh, who, after 1870, discovered a great number of bird remains in the Cretaceous strata of North America. The most important result is the proof that, until the end of the Cretaceous epoch, most, if not all, birds were still possessed of teeth (see ODONTORNITHES).

The oldest known bird is the _Archaeopteryx_ (q.v.), of the upper Oolite in Bavaria. The imprints in the enormously older new red sandstone or Lower Trias of Connecticut, and originally named _Ornithichnites_, belong to Dinosaurian Reptiles.

A wide gap separates _Archaeopteryx_ from the next order of fossil birds of the Cretaceous epoch, and, since freshwater deposits of that age are rare, bird remains are uncommon. Many bones formerly referred to birds have since proved to belong to Pterodactyls, e.g. _Cimoliornis_ from the English Chalk. But in 1858 were discerned in the Upper Greensand of Cambridgeshire remains which are now known as _Enaliornis_. W. Dames has described bones from the Chalk of southern Sweden under the name of _Scaniornis_, probably allied to _Palaelodus_. From the Cretaceous rocks of North America a large number of birds have been described by O.C. Marsh. Of these the most interesting are _Ichthyornis_ (= _Graculavus_) and _Hesperornis_, from the Cretaceous shales of Kansas. They were placed by Marsh in a distinct subclass of birds, _Odontornithes_ (q.v.). Probably all birds of Cretaceous age were still possessed of teeth. _Baptornis_, another of Marsh's genera, seems to be allied to _Enaliornis_, _Palaeotringa_ and _Talmatornis_, were by him referred to Limicoline and Passerine birds. _Laornis_ from the Cretaceous marls of New Jersey was as large as a swan.

The lower Eocene has furnished a greater number of bird bones. Some of the largest are those of _Gastornis_, with three species from France, Belgium and England. Much difference of opinion obtains as to the affinities of these birds, which were far larger than an ostrich; they were undoubtedly incapable of flight and there are indications of teeth in the upper jaw. Provisionally this genus has been grouped with the Ratitae, which at any rate are a heterogenous assembly. Sir R. Owen's _Dasornis_, of the London Clay, known from an imperfect cranium, and E.D. Cope's _Diatryma_ of New Mexico, based upon a gigantic metatarsus, may also belong there. The London Clay of South England has likewise supplied some long upper arm bones, _Argillornis_. The most remarkable specimen is a skull, _Odontopteryx toliapicus_ (figs. 17, 18); the edges of the jaws were serrated like those of certain tortoises. The character of this skull and the compound rhamphotheca (known by the imprints left upon the jaws) indicate affinities with the Steganopodes. Remnants of a heron-like bird, _Proherodius_, of a gull-like creature, _Halcyornis_, a raptorial _Lithornis_; and a supposed Passerine from Glarus in Switzerland, called _Protornis = Osteornis_, complete the list.

The upper Eocene has yielded many birds, most of which are at least close forerunners of recent genera, the differentiation into the leading orders and families being already well marked, e.g. Gallinaceous birds, stork- and crane-like waders, rails, birds of prey, cormorants, &c. Especially numerous bones have been found in the Paris basin, chiefly described by G. Cuvier, F.L.P. Gervais, E. Blanchard, and above all by A. Milne-Edwards, and in the equivalent beds of Hampshire. Others have been discovered in Wyoming; a giant penguin, _Palaecudyptes_, is known from New Zealand, and _Palaeospheniscus_ from Patagonia. The Miocene has yielded by far the greatest number of bird-bones, including even eggs and imprints of feathers. For instance, from the lower Miocene beds of Allier and Puy-de-Dome Milne-Edwards has described about 50 species. Of these _Palaelodus_ was an ancestral flamingo, but with shorter legs; _Limnatornis_ is referred to the hoopoes. The existing genera include _Anas, Aquila, Bubo, Columba, Cypselus, Lanius, Picus, Phalacrocorax, Sula_, &c. Very interesting is the fact that _Serpentarius, Psittacus_ and _Trogon_ are amongst this list of birds, which are now restricted to the tropics. A similarly mixed avifauna has been found in the mid-Miocene beds of various other parts of France, Germany and Italy. In Colorado and New Mexico Marsh has detected bones of _Meleagris, Puffinus, Sula_ and _Uria_, all existing genera; but the first is especially suggestive, since it is one of the most characteristic forms of the New World.

Here may be interpolated a short account of the very peculiar avifauna found in the Tertiary strata of Santa Cruz in Patagonia. Instead of the age of lower Eocene, as had been stated originally, these beds are not older than mid-Miocene, and not a few of the bones are of a much younger, even latest Tertiary date. Discovered, and partly described, by F. Ameghino, the bones have been sumptuously monographed by F.P. Moreno and A. Mercerat, who proposed for them the name of _Stereornithes_, a new order of birds, mostly gigantic in size, and said to combine the characters of Anseres, Herodiones and Accipitres. But the whole mass of bones is in hopeless disorder, apparently without any record of association. At any rate, the "_Stereornithes_," accepted as such in Bronn's _Thierreich_, and in Newton's _Dictionary of Birds_, had to be dissolved as an unnatural, haphazard assembly. Many of these birds, to judge from the enormous size of their hind-limbs, were undoubtedly flightless, e.g. _Brontornis_, and remind us of the Eocene _Gastornis_ of Europe. _Phororhacos_, the most extraordinary of all, belongs to the Gruiformes, perhaps also _Pelecyornis_ and _Liornis_. On the other hand, the late Tertiary _Dryornis_ is a member of the Cathartae or American vultures, and _Mesembriornis_, likewise of late Tertiary date, is a close forerunner of the recent genus _Rhea_.

Pliocene remains are less numerous than those of the Miocene. From Pikermi in Greece is known a _Gallus_, a _Phasianus_ and a large _Grus_. From Samos a large stork, _Amphipelargus_, and a typical _Struthio_; from the Sivalik Hills on the southern flanks of the Himalayas also an ostrich, and another Ratite with three toes, _Hypselornis_, as well as _Leptoptilus, Pelecanus_ and _Phalacrocorax_. The fossil egg of a struthious bird, _Struthiolithus_, has been found near Cherson, south Russia, and in north China. The Suffolk Crag has yielded the unmistakable bones of an albatross, _Diomedea_.

Most Pleistocene birds are generically, even specifically, identical with recent forms; some, however, have become extinct, or they have become exterminated by man. A great number of birds' bones have been found in caves, and among them some bearing marks of human workmanship. In France we have a large and extinct crane, _Grus primigenia_, but more interesting are the numerous relics of two species, the concomitants even now of the reindeer, which were abundant in that country at the period when this beast flourished there, and have followed it in its northward retreat. These are the snowy owl, _Nyctea scandiaca_, and the willow-grouse, _Lagopus albus_. A gigantic swan, _Cygnus falconeri_, is known from the Zebug cavern in Malta. From caves of Minas Geraes in Brazil, O. Winge has determined at least 126 species, of which nearly all still survive in the country. Kitchen-middens of England, Ireland and Denmark reveal the existence of the capercally, _Tetrao urogallus_, and of the great auk or gare-fowl, _Alca impennis_; both species long since vanished from those countries. In the fens of East Anglia have been found two humeri, one of them immature, of a true _Pelecanus_, a bird now no longer inhabiting middle Europe.

Until a very recent epoch there flourished in Madagascar huge birds referable to the Ratitae, e.g. _Aepyornis maximus_, which laid enormous eggs, and not unnaturally recalls the mythical "roc" that figures so largely in Arabian tales. New Zealand has also yielded many flightless birds, notably the numerous species and genera of _Dinornithidae_, some of which survived into the 19th century (see MOA); _Pseudapteryx_ allied to the _Kiwi; Cnemiornis_, a big, flightless goose; _Aptornis_ and _Notornis_, flightless rails; and _Harpagornis_, a truly gigantic bird of prey with tremendous wings and talons.

From a tracing by M.A. Milne-Edwards of the original drawing in a MS. Journal kept during Wolphart Harmanszoon's voyage to Mauritius (A.D. 1601-1602), _penes_ H. Schlegel (_Proc. Zool. Soc._ 1875, p. 350). Reduced.]

It is, of course, quite impossible, in a survey of extinct birds, to divide them into those which are _bona fide_ fossil, sub-fossil, recently extirpated and partially exterminated. Nor is it possible, except in a few cases, to decide whether they have come to an end through the agency of man or through so-called natural causes. Like other creatures birds have come, some to flourish and stay, others to die out.

Mauritius is famous for the dodo, killed off by man; there was also a curiously crested parrot, _Lophopsittacus_ (fig. 19). In the Mare aux Songes have been found the bones of another parrot, of ducks, pigeons, rails, herons, geese and of a dwarf darter, _Plotus nanus_, all sub-fossil, now extinct. Very interesting is _Aphanapteryx_ (fig. 20), a long-billed, flightless rail, practically the same as _Erythromachus_ of Rodriguez and _Diaphorapteryx_ of Chatham Island. Reunion possessed the peculiar starling, _Fregilupus_. Rodriguez was inhabited by _Pezophaps_, the solitaire, _Necropsittacus_ and _Palaeornis exsul_, which is now probably extinct. The Antilles tell a similar tale. The great auk, once common on the British coasts, those of Denmark, the east coast of North America, then restricted to those of Newfoundland, Greenland and Iceland, has been killed by man, and the same fate has overtaken the Labrador duck, the Phillip Island parrot, _Nestor productus_, and the large cormorant of Bering Island, _Phalacrocorax perspicillatus_; and how long will the flightless cormorant, _Ph. harrisi_ of the Galapagos, survive its quite recent discovery?

AUTHORITIES.--A. Milne-Edwards, _Recherches anatomiques et
paleontologiques pour servir a l'histoire des oiseaux fossiles de la
France_ (Paris, 1867-1868); F.P. Moreno and A. Mercerat, _Catalogo de
los Pajaros fosiles de la Republica Argentina. Anales Mus. La Plata_,
1891, 21 pls.; O.C. Marsh, _Odontornithes: A monograph of the Extinct
Toothed Birds of North America_ (New Haven, Conn., 1880); R. Lydekker,
article "Fossil Birds," in A. Newton's _Dictionary of Birds_ (London,
1893); Cat. Foss. Birds, Brit. Museum, 1891; K. v. Zittel, _Handbuch
der Palaontologie_, i. 3 (1887-1890); C.W. Andrews, "On the Extinct
Birds of Patagonia," _Tr. Zool. Soc_. xv., 1899, pp. 55-86, pls.
14-17.

C. GEOGRAPHICAL DISTRIBUTION

The study of the extinct organisms of any country leads to a proper appreciation of its existing flora and fauna; while, on the other hand, a due consideration of the plants and animals which may predominate within its bounds cannot fail to throw more or less light on the changes it has in the course of ages undergone. That is to say, the distribution of forms in time is a subject so much connected with the distribution of forms in space, that the one can hardly be separated from the other. Granting this is a general truth, it must yet be acknowledged as a special fact, that in fossil birds we have as yet but scanty means of arriving at any precise results which will justify bold generalization in the matter of avine distribution. Remains of extinct birds are, compared with those of other classes of vertebrates, exceedingly scarce, and these have been found in very few, widely separated countries. The great problems involved in the study of geographical distribution must therefore be based mainly upon the other classes, both vertebrate and invertebrate, which, moreover, enjoy less great facilities of locomotion than the birds.