Chapter XX: Part 20

The birch is one of the most wide-spread and generally useful of forest trees of Russia, occurring in that empire in vast forests, in many instances alone, and in other cases mingled with pines, poplars and other forest trees. The wood is highly valued by carriage-builders, upholsterers and turners, on account of its toughness and tenacity, and in Russia it is prized as firewood and a source of charcoal. A very extensive domestic industry in Russia consists in the manufacture of wooden spoons, which are made to the extent of 30,000,000 annually, mostly of birch. Its pliant and flexible branches are made into brooms; and in ancient Rome the fasces of the lictors, with which they cleared the way for the magistrates, were made up of birch rods. A similar use of birch rods has continued among pedagogues to times so recent that the birch is yet, literally or metaphorically, the instrument of school-room discipline. The bark of the common birch is much more durable, and industrially of greater value, than the wood. It is impermeable to water, and is therefore used in northern countries for roofing, for domestic utensils, for boxes and jars to contain both solid and liquid substances, and for a kind of bark shoes, of which it is estimated 25 millions of pairs are annually worn by the Russian peasantry. The jars and boxes of birch bark made by Russian peasants are often stamped with very effective patterns. By dry distillation the bark yields an empyreumatic oil, called _diogott_ in Russia, used in the preparation of Russia leather; to this oil the peculiar pleasant odour of the leather is due. The bark itself is used in tanning; and by the Samoiedes and Kamchatkans it is ground up and eaten on account of the starchy matter it contains. A sugary sap is drawn from the trunk in the spring before the opening of the leaf-buds, and is fermented into a kind of beer and vinegar. The whole tree, but especially the bark and leaves, has a very pleasant resinous odour, and from the young leaves and buds an essential oil is distilled with water. The leaves are used as fodder in northern latitudes.

The species which belong peculiarly to America (_B. lenta, excelsa, nigra, papyracea_, &c.) are generally similar in appearance and properties to _B. alba_, and have the same range of applications. The largest and most valuable is the black birch (_B. lenta_) found abundantly over an extensive area in British North America, growing 60 to 70 ft. high and 2 to 3 ft. in diameter. It is a wood most extensively used for furniture and for carriage-building, being tough in texture and bearing shocks well, while much of it has a handsome grain and it is susceptible of a fine polish. The bark, which is dark brown or reddish, and very durable, is used by Indians and backwoodsmen in the same way as the bark of _B. alba_ is used in northern Europe.

The canoe or paper birch (_B. papyracea_) is found as far north as 70 deg. N. on the American continent, but it becomes rare and stunted in the Arctic circle. Professor Charles Sprague Sargent says: "It is one of the most widely distributed trees of North America. From Labrador it ranges to the southern shores of Hudson's Bay and to those of the Great Bear Lake, and to the valley of the Yukon and the coast of Alaska, forming with the aspen, the larch, the balsam poplar, the banksian pine, the black and white spruces and the balsam fir, the great subarctic transcontinental forest; and southward it ranges through all the forest region of the Dominion of Canada and the northern states." It is a tree of the greatest value to the inhabitants of the Mackenzie river district in British North America. Its bark is used for the construction of canoes, and for drinking-cups, dishes and baskets. From the wood, platters, axe-handles, snow-shoe frames, and dog sledges are made, and it is worked into articles of furniture which are susceptible of a good polish. The sap which flows in the spring is drawn off and boiled down to an agreeable spirit, or fermented with a birch-wine of considerable alcoholic strength. The bark is also used as a substitute for paper. A species (_B. Bhojputtra_) growing on the Himalayan Mountains, as high up as 9000 ft., yields large quantities of fine thin papery bark, extensively sent down to the plains as a substitute for wrapping paper, for covering the "snakes" of hookahs and for umbrellas. It is also said to be used as writing paper by the mountaineers; and in Kashmir it is in general use for roofing houses.

BIRCH-PFEIFFER, CHARLOTTE (1800-1868), German actress and dramatic writer, was born at Stuttgart on the 23rd of June 1800, the daughter of an estate agent named Pfeiffer. She received her early training at the Munich court theatre, and in 1818 began to play leading tragic roles at various theatres. In 1825 she married the historian Christian Birch of Copenhagen, but continued to act. From 1837 to 1843 she managed the theatre at Zuerich. In 1844 she accepted an engagement at the royal theatre in Berlin, to which she remained attached until her death on the 24th of August 1868. Her intimate knowledge of the technical necessities of the stage fitted her for the successful dramatization of many popular novels, and her plays, adapted and original, make twenty-three volumes, _Gesammelte dramatische Werke_ (Leip. 1863-1880). Many still retain the public favour. Her novels and tales, _Gesammelte Novellen und Erzaehlungen_, were collected in three volumes (Leip. 1863-1865).

Her daughter, WILHELMINE VON HILLERN (b. 1836), born at Munich, went on the stage, but retired upon her marriage in 1857. After 1889 she lived in Oberammergau and won a reputation as a novelist. Her most popular works are _Ein Arzt der Seele_ (1869, 4th ed. 1886); and _Die Geier-Wally_ (1883), which was dramatized and translated into English as _The Vulture Maiden_ (Leip. 1876).

BIRD, the common English name for feathered vertebrates, members of the class _Aves_. The word in Old Eng. is _brid_ and in Mid. Eng. _byrd_ or _bryd_, and in early uses meant the young or nestlings only. It is partly due to this early meaning that the derivation from the root of "brood" has been usually accepted; this the _New English Dictionary_ regards as "inadmissible." The word does not occur in any other Teutonic language. As a generic name for the feathered vertebrates "bird" has replaced the older "fowl," a common Teutonic word, appearing in German as _Vogel_. "Bird," when it passed from its earliest meaning of "nestlings," seems to have been applied to the smaller, and "fowl" to the larger species, a distinction which was retained by Johnson. In modern usage "fowl," except in "wild-fowl" or "water-fowl," is confined to domestic poultry.

The scope of the anatomical part of the following article is a general account of the structure of birds (_Aves_) in so far as they, as a class, differ from other vertebrates, notably reptiles and mammals, whilst features especially characteristic, peculiar or unique, have been dwelt upon at greater length so far as space permitted. References to original papers indicate further sources of information. For a comprehensive account the reader may be referred to Prof. M. Fuerbringer's enormous work _Untersuchungen zur Morphologie und Systematik der Voegel_, 4to., 2 vols. (1888); H.G. Bronn's _Klassen und Ordnungen des Thierreichs_, vol. vi., "Aves," Leipzig, completed 1893 by Gadow; and A. Newton's _Dictionary of Birds_, London, 1896. For the history of the classification of birds see the article ORNITHOLOGY, where also the more important ornithological works are mentioned. EGG, FEATHER (including Moult), MIGRATION, &c., also form separate articles to which reference should be made. In this article (A) the general anatomy of birds is discussed, (B) fossil birds, (C) the geographical distribution of birds, (D) the latest classification of birds.

A. ANATOMY OF BIRDS

1. _Skeleton_.

_Skull._--When W.K. Parker wrote the account of the skull in the article BIRDS for the 9th edition of the _Encyclopaedia Britannica_, he had still to wrestle with the general problem of the composition and evolution of the skull. That chapter of comparative anatomy (together with other anatomical details, for which see the separate articles) is now dealt with in the article SKULL; here only the most avine features are alluded to, and since some of Parker's original illustrations have been retained, the description has been shortened considerably.

One general feature of the adult bird's skull is the almost complete
disappearance of the sutures between the bones of the cranium proper,
whilst another is the great movability of the whole palatal and other
suspensorial apparatus. The occipital condyle (fig. 1) is a single
knob, being formed almost wholly by the basioccipital, while the
lateral occipitals (often perversely called exoccipitals) take but
little share in it. Part of the membranous roof between the
supra-occipital and parietal bones frequently remains unossified and
presents in the macerated skull a pair of fontanelles. The squamosals
form the posterior outer margin of the orbits and are frequently
continued into two lateral downward processes across the temporal
fossa. One of these, the _processus orbitatis posterior_, often
combines with an outgrowth of the alisphenoid, and may be, e.g. in
cockatoos, continued forwards to the lacrymal bone, so as to form a
complete infraorbital bridge. The posterior, so-called _processus
Zygomaticus_ is very variable; in many Galli it encloses a foramen by
distally joining the orbital process. The ethmoid frequently appears
on the dorsal surface between the frontals. There are three periotic
bones (pro-, epi-, opisth-otic). The prooetic encloses between it and
the lateral occipital the fenestra ovalis, into which fits the
columella of the ear. The epiotic is often small, ossifies
irregularly, and fuses with the supra-occipital. The opisthotic lies
between the epiotic and the lateral occipital with which it ultimately
fuses; in some birds, e.g. in _Larus_, it extends far enough to help
to bound the foramen magnum. The basisphenoids are ventrally overlaid,
and later on fused with, a pair of membrane bones, the basi-temporals,
homologous in part with the parasphenoid of lower vertebrates. They
contribute to the formation of the auditory meatus, and of the right
and left carotid canals which accompany the eustachian tubes. In many
birds the basisphenoids send out a pair of basipterygoid processes by
which they articulate with the pterygoids. Dorso-laterally the
basisphenoid is joined by the alisphenoid, which forms most of the
posterior wall of the orbit. The orbito-sphenoids diverge only
posteriorly, otherwise they are practically unpaired and form the
median interorbital septum, which is very large in correlation with
the extraordinary size of the eyeballs.,

bo, Basi-occipital.
bt, Basi-temporal.
eo, Opisthotic.
f, Frontal.
fm, Foramen magnum.
fo, Fontanella.
oc, Occipital condyle.
op, Opisthotic.
p, Parietal.
pf, Post-frontal.
sc, Sinus canal in supra-occipital.
so, Supra-occipital.
sq, Squamosal.
8, Exit of vagus nerve.]

as, Alisphenoid. pa, Palatine.
bo, Basi-occipital. pf, Post-frontal.
bt, Basi-temporal. pg, Pterygoid.
dpx, Dentary process of pn, Prenasal cartilage.
premaxilla. ppx, Palatine process of pre-
eo, Opisthotic. maxillary.
eu, Eustachian tube. prp, Pterygoid process of sphenoid.
f, Frontal. qj, Quadratojugal.
fm, Foramen magnum. so, Supra-occipital.
j, Jugal. sq, Squamosal.
l, Lacrymal. ty, Tympanic cavity.
mx, Maxilla. v, Vomer.
mxp, Maxillo-palatine process. 8, Exit of vagus nerve.
oc, Occipital condyle. 9, Exit of hypoglossal nerve.]

eo, Lateral occipital. npx, Nasal process of premaxillary.
eth, Ethmoid. p, Parietal.
f, Frontal. pf, Post-frontal.
j, Jugal. px, Premaxilla.
l, Lacrymal. qj, Quadratojugal.
n, Nostril. so, Supra-occipital.
np, Upper process of nasal. sq, Squamosal.]

Prefrontal bones are absent; post-frontals are possibly indicated by a
frequently occurring separate centre of ossification in the
post-orbital process, to which the frontals always contribute. The
lacrymal is always present, and perforated by a glandular duct.
Attached to it or the neighbouring frontal is often a supraorbital;
infraorbitals occur also, attached to the jugal or downward process of
the lacrymal. The nasals were used by A.H. Garrod to distinguish the
birds as holorhinal (fig. 2) where the anterior margin of the nasal is
concave, and schizorhinal where this posterior border of the outer
nares is continued backwards into a slit which extends beyond the
frontal processes of the premaxilla. Many birds possess a more or less
well developed cross-joint in front of the frontals and lacrymals,
perhaps best developed in _Anseres_ and _Psittaci_. Owing to this
joint the whole upper beak can be moved up and down with extra
facility, according to the shoving forwards or backwards of the
palato-pterygo-quadrate apparatus which moves sledge-like upon the
cranial basis. The premaxilla is always unpaired, but each half has
three long processes directed backwards; one fuses with the maxillary
bone, another helps to form the anterior part of the palate, while the
third, together with its fellow, forms the "culmen" and extends
backwards to the frontals, or rather to the ethmoid which there crops
up on the surface. The maxillaries (fig. 3) have besides others, a
maxillo-palatine process directed inwards in a transverse horizontal
direction. The palatines are long, always fused anteriorly with the
premaxilla, and frequently with the maxillo-palatine processes;
posteriorly they slide upon the presphenoidal rostrum, and articulate
in most birds with the pterygoids; they form the greater part of the
palatal roof and border the choanae or inner nares. Between these,
resting vertically upon the rostrum, appears the vomer; very variable
in shape and size, often reduced to a mere trace, as in the Galli, or
even absent, broken up into a pair of tiny splints in Pici.

The taxonomic importance of the configurations of the palate was first
pointed out by J. de Cornay. T.H. Huxley, in 1868, divided the
carinate birds into Dromaeo-, Schizo-, Desmo-, and Aegithognathae, an
arrangement which for many years had a considerable influence upon
classification. However, subsequent additions and corrections have
detracted much from its value, especially when it became understood
that the above sub-orders are by no means natural groups.
_Dromaeognathae_ have a struthious palate, with a broad vomer meeting
in front the broad maxillo-palatal plates, while behind it reaches the
pterygoids. The only representatives are the Tinamous.
_Schizognathae_, e.g. fowls (fig. 4), pigeons, gulls, plovers, rails
and penguins, have the vomer pointed in front while the
maxillo-palatines are free, leaving a fissure between the vomer and
themselves. The schizognathous formation is doubtless the most
primitive, and its representatives form a tolerably natural assembly.
_Desmognathae_ (fig. 5) were supposed to have the maxillo-palatines
united across the middle line, either directly or by the
intermediation of ossifications in the nasal septum. This is a
hopeless assembly. Parker and Fuerbringer have demonstrated that
desmognathism has been produced in half a dozen ways, implying
numerous cases of convergence without any nearer relationship than
that they are all derived from some schizognathous group or other. The
_Aegithognathae_, meant to comprise the passeres, woodpeckers and
swifts, &c., are really schizognathous but with a vomer which is
broadly truncated in front.

bo, Basi-occipital. pg, Pterygoid.
bt, Basi-temporal. prp, Pterygoid process of sphenoid.
eo, Lateral occipital. px, Premaxilla.
eu, Eustachian tube. q, Quadrate.
ic, Internal carotid. qj, Quadratojugal.
j, Jugal. rbs, Rostrum of basisphenoid.
l, Lacrymal. so, Supra-occipital.
mx, Maxilla. v, Vomer.
mxp, Maxillo-palatine process. 8, Exit of vagus nerve.
oc, Occipital condyle. 9, Exit of hypoglossal nerve.
pa, Palatine.
pf, Post-frontal. (After W.K. Parker)]

bt, Basi-temporal. pg, Pterygoid.
bpg, Basi-pterygoid. px, Premaxilla.
eo, Lateral occipital. pto, Prootic.
f, Frontal. q, Quadrate.
fm, Foramen magnum. qj, Quadratojugal.
j, Jugal. sn, Nasal septum
l, Lacrymal. so, Supra-occipital
mpg, Mesopterygoid process ty, Tymapanic cavity.
of W.K. Parker. v, Vomer
mx, Maxillary. 8, Exit of vagus nerve.
mxp, Maxillo-palatine process. 9. Exit of hypoglossal nerve.
op, Opisthotic.
pa, Palatine. (After W.K. Parker.)]

The remainder of the appendicular skeleton (fig. 6) of the head
requires little description. The maxillaries are connected with the
distal anterior corner of the quadrate by the thin, splint-like jugal
and quadratojugal. The quadrate is invariably a conspicuous bone and
movably articulating with the cranium and by a special process with
the pterygoid. The mandible is composed of several bones as in
reptiles. The os articulare bears on its inner side the inner
mandibular process which serves for the insertion of part of the
digastric muscle or opener of the mouth; another portion of this
muscle is attached to the os angulare, which frequently forms a
posterior mandibular process. The greater part of the under-jaw is
formed by the right and left dentaries, which in all recent birds are
fused together in front. Supra-angular and coronoid splint-bones serve
for the insertion of part of the temporal or masseter muscle.
Additional splints rest on the inner side of the jaw. Like the
crocodiles, birds possess a _siphonium_, i.e. a membranous, or
ossified, tube which rises from a pneumatic foramen in the os
articulare, on the median side of the articulation, and passes upwards
between the quadrate and lateral occipital bone, opening into the
cavity of the middle ear.

a, Angular of mandible. pa, Palatine.
ar, Articular. pap, Posterior angular process of
bt, Basi-temporal. mandible.
d, Dentary. pe, Ethmoid.
eo, Lateral occipital. pf, Post-frontal.
eth, Ethmoid. pg, Pterygoid.
f, Frontal. ps, Pre-sphenoid.
iap, Interangular process px, Premaxilla.
of mandible q, Quadrate.
ios, Interorbital septum. qj, Quadratojugal.
j, Jugal. sa, Supra-angular or coronoid.
l, Lacrymal. so, Supra-occipital.
mx, Maxillar. sq, Squamosal.
n, Nasal. ty, Tympanic cavity.
os, Orbito-sphenoid. v, Vomer.
p, Parietal. 1, Exit of olfactory nerve.]

The _Hyoid apparatus_ is, in its detail, subject to many variations in
accord with the very diverse uses to which the tongue of birds is
put. It consists of (1) the basihyal variously called copula, or
corpus linguae, or unpaired middle portion. (2) The urohyal likewise
unpaired, rested ventrally on the larynx. (3) The os entoglossum
originally paired, but coalescing into an arrow-headed piece, attached
to the anterior end of the basihyal and lodged in the tongue proper.
It is homologous with the distal ends of the ceratohyals or ventral
elements of the hyoidean or second visceral arch. The dorsal or
hyomandibular portion of this same arch is transformed into the
auditory chain, ending in the fenestra ovalis. (4) A pair of
thyrohyals, homologous with the posterior hyoid horns of mammals, i.e.
third visceral or first branchial arch. As the most developed pair in
birds they are commonly, although wrongly, called the hyoid horns.
They articulate upon facets of the hinder outer corners of the
basihyal.

c.h, Ceratohyals (confluent).
b.h, The so-called basihyal, answering to the first basibranchial of
a fish.
b.br, Basibranchial, or urohyal, answering to the rest of the
basibranchial series.
c.br, e.br, together form the thyrohyal, answering to the first
cerato- and epibranchials.]

The vertebrae are stereospondylous, the centrum or body and the arch
being completely fused into one mass, leaving not even a neuro-central
suture. The arch alone sends out processes, viz. the spinous process,
the anterior and posterior oblique (commonly called pre- and
post-zygapophyses), and the transverse processes. The latter
articulate with the tuberculum of the corresponding rib, while the
capitulum articulates by a knob on the side of the anterior end of the
centrum. In the cervical region the ribs are much reduced, fused with
their vertebrae and enclosing the transverse canal or foramen. When
the vertebrae are free their centra articulate with each other by
complicated joints, exhibiting four types. (1) Amphicoelous; each end
of the centrum is concave; this, the lowest condition, is embryonic,
but was retained in _Archaeopteryx_ and in the thoracic vertebrae of
_Ichthyornis_. (2) Procoelous, concave in front; only in the atlas,
for the reception of the occipital condyle. (3) Opisthocoelous, or
concave behind, only occasionally found in the thoracic region, e.g.
Sphenisci. (4) Heterocoelous (fig. 8) or saddle-shaped; the anterior
surface is concave in a transverse, but convex in a vertical
direction, which on posterior surface shows the conditions reversed.
This is the most perfect arrangement attained by the vertebral column,
and is typical of, and restricted to, birds. The intervertebral joints
are further complicated by the interposition of a cartilaginous or
fibrous pad or ring. This pad varies much; it is morphologically the
homologue of the pair of basiventral elements which by their lateral
extension give origin to the corresponding ribs. Later those pads fuse
with the anterior end of the centrum of the vertebra to which they
belong; where the vertebral column is rendered inflexible, the disks
are ossified with the centra and all trace of them is lost. Sometimes
the pad is reduced to a ventral semi-ring or meniscus; it retains its
largest almost original shape and size in the second vertebra, the
axis or epistropheus, where it forms a separately ossifying piece
which connects, and cooessifies with, the odontoid process (the centrum
of the atlas) and the centrum of the second vertebra. Sometimes the
ventral portions of these pads form paired or unpaired little
ossifications, then generally described as intercentra; such are not
uncommon on the tail. The atlas is composed of three pieces; a pair of
lateral elements (the right and left dorsal arch pieces) joining above
the spinal cord, and a ventral piece equivalent to the first
basiventral elements, i.e. serially homologous with the intervertebral
pads. In the adults the atlas forms a more or less solid ring. A
remnant of the _chorda dorsalis_ and its sheath persists as the
_ligamentum suspensorium_ between the central portions of the
successive vertebrae.

In birds we distinguish between the following regions of the _axial
skeleton_. (1) Cervical vertebrae, or those between the skull and the
first vertebra which is connected with the sternum by a pair of
complete ribs. The last 1 to 5 of these vertebrae have movable ribs
which do not reach the sternum, and are called cervico-dorsals. (2)
Dorsals, those which begin with the first thoracic rib, and end at the
last that is not fused with the ilium. The term "lumbar" vertebrae is
inapplicable to birds. (3) Pelvic, all those which are fused with the
iliac portion of the pelvis, generally a considerable number. (4)
Caudal, those which are not connected with the pelvis. It is to be
noted that often no absolute line of demarcation can be drawn in
regard to these regions, their definitions being rather convenient
than morphological.

In comparison with all other vertebrates the number of neck-vertebrae
of the birds is considerably increased; the lowest number, 14 to 15,
is that of most Passeres and many other Coraciomorphae; the largest
numbers, 20 or 21, are found in the ostrich, 23 in _Cygnus olor_ and
25 in the black swan. Dorsal vertebrae frequently have a ventral
outgrowth of the centrum; these hypapophyses may be simple vertical
blades, _|_-shaped, or paired knobs; they serve for the attachment of
the thoracic origin of the longus collianticus muscle, reaching their
greatest development in Sphenisci and Colymbidae. In many birds some
of the thoracic vertebrae are more or less cooessified, in most pigeons
for instance the 15th to 17th; in most Galli the last cervical and the
next three or four thoracics are coalesced, &c. The pelvic vertebrae
include of course the sacrum. There are only two or three vertebrae
which are equivalent to those of the reptiles; these true sacrals are
situated in a level just behind the acetabulum; as a rule between
these two primary sacral vertebrae issues the last of the spinal
nerves which contributes to the composition of the sciadic plexus.
These true sacrals alone are connected with the ilium by processes
which are really equivalent to modified ribs; but the pelvis of birds
extends considerably farther forwards and backwards, gradually coming
into contact with other vertebrae, which in various ways send out
connecting transverse processes or buttresses, and thus become pre-
and post-sacral vertebrae (fig. 9). The most anterior part of the
ilium often overlaps one or more short lumbar ribs and fuses with
them, or even a long, complete thoracic rib. Similarly during the
growth of the bird the posterior end of the ilium connects itself with
the transverse processes of vertebrae which were originally free, thus
transforming them from caudals into secondary post-sacrals.
Individual, specific and generic variations are frequent.

The last six or seven caudal vertebrae coalesce into the pygostyle, an
upright blade which carries the rectrices. Such a pygostyle is absent
in _Archaeopteryx, Hesperornis, Tinami_ and _Ratitae_, but it occurs
individually in old specimens of the ostrich and the kiwi. In
_Ichthyornis_ it is very small. In all the _Neornithes_ the total
number of caudal vertebrae, inclusive of those which coalesce, is
reduced to at least 13.

_Sternum_ (figs. 10 and 11).--Characteristic features of the sternum
are the following. There is a well-marked _processus lateralis
anterior_ (the right and left together equivalent to the mammalian
manubrium), which is the product of two or three ribs, the dorsal
parts of which reduced ribs remain as cervico-dorsal ribs. Then
follows the rib-bearing portion and then the _processus lateralis
posterior_; this also is the product of ribs, consequently the right
and left processes together are equivalent to the xiphoid process or
xiphisternum of the mammals. The lateral process in most birds sends
out an outgrowth, directed out and upwards, overlapping some of the
ribs, the _processus obliquus_. The median and posterior extension of
the body of the sternum is a direct outgrowth of the latter, therefore
called meta-sternum. The anterior margin of the sternum, between the
right and left anterior lateral processes receives in sockets the feet
of the coracoids. Between them arises a median crest, which varies
much in extent and composition, and is of considerable taxonomic
value. It is represented either by a _spina interna_ or by a _spina
externa_, or by both, or they join to form a _spina communis_ which is
often very large and sometimes ends in a bifurcation. Eventually, when
the right and left feet of the coracoids overlap each other, the
anterior sternal spine contains a foramen. The keel, or _carina
sterni_, is formed as a direct cartilaginous outgrowth of the body of
the sternum, ossifying from a special centre. This keel is much
reduced in the New Zealand parrot, _Stringops_, less in various
flightless rails, in the dodo and solitaire. It is absent in the
Ratitae, which from this feature have received their name, but
considerable traces of a cartilaginous keel occur in the embryo of the
ostrich, showing undeniably that the absence of a keel in the recent
bird is not a primitive, fundamental feature. The keel has been lost,
and is being lost, at various epochs and by various groups of birds.
The swimming _Hesperornis_ (see ODONTORNITHES) was also devoid of such
a structure. In many birds the spaces between the meta-sternum and the
posterior processes and again the spaces between this and the oblique
process are filled up by proceeding ossification and either remain as
notches, or as fenestrae, or they are completely abolished so that the
breastbone is turned into one solid more or less oblong plate.

_Shoulder Girdle._--Scapula, coracoid and clavicle, meet to form the
foramen triosseum, through which passes the tendon of the
_supracoracoideus_, or _subclavius_ muscle to the _tuberculum
superius_ of the humerus. The coracoid is one of the most
characteristic bones of the bird's skeleton. Its upper end forms the
acrocoracoid process, against the inner surface of which leans the
proximal portion of the clavicle. From the inner side of the neck of
the coracoid arises the precoracoidal process, the remnant of the
precoracoid. Only in the ostrich this element is almost typically
complete, although soon fused at either end with the coracoid. Near
the base of the precoracoidal process is a small foramen for the
passage of the _nervus supracoracoideus_. In most birds the feet of
the coracoids do not touch each other; in some groups they meet, in
others one overlaps the other, the right lying ventrally upon the
left. The scapula is sabre-shaped, and extends backwards over the
ribs, lying almost parallel to the vertebral column. This is a
peculiar character of all birds. The clavicles, when united, as usual,
form the furcula; mostly the distal median portion is drawn out into a
hypocleidium of various shape. Often it reaches the keel of the
sternum, with subsequent syndosmosis or even synostosis, e.g. in the
gannet. In birds of various groups the clavicles are more or less
degenerated, the reduction beginning at the distal end. This condition
occurs in the Ratitae as well as in the well-flying _Platyrcecinae_
amongst parrots.

h, Humerus.
r, Radius.
u, Ulnar.
r', u', Radial and ulnar carpal bones; with the three digits I.,
II., III.]

The _fore-limb_ or _wing_ (fig. 12); highly specialized for flight,
which, initiated and made possible mainly by the strong development of
quill-feathers, has turned the wing into a unique organ. The humerus
with its crests, ridges and processes, presents so many modifications
characteristic of the various groups of birds, that its configuration
alone is not only of considerable taxonomic value but that almost any
genus, excepting, of course, those of Passeres, can be "spotted" by a
close examination and comparison of this bone. When the wing is folded
the long glenoid surface of the head of the humerus is bordered above
by the _tuberculum externum_ or _superius_, in the middle and below by
the _tuberculum medium_ or _inferius_ for the insertion of the
_coraco-brachialis posterior_ muscle. From the outer tuberculum
extends the large _crista superior_ (insertion of _pectoralis major_
and of _deltoideus major_ muscles). The ventral portion of the neck is
formed by the strong _crista inferior_, on the median side of which is
the deep _fossa subtrochanterica_ by which air sacs enter the humerus.
On the outer side of the humerus between the head and the _crista
inferior_ is a groove lodging one of the coraco-humeral ligaments. The
distal end of the humerus ends in a trochlea, with a larger knob for
the ulna and a smaller oval knob for the radius. Above this knob is
often present an ectepicondylar process whence arise the tendons of
the ulnar and radial flexors. The radius is the straighter and more
slender of the two forearm bones. Its proximal end forms a shallow cup
for articulation with the outer condyle of the humerus; the distal end
bears a knob which fits into the radial carpal. The ulna is curved and
rather stout; it articulates with both carpal bones; the cubital
quills often cause rugosities on its dorsal surface. Of wrist-bones
only two remain in the adult bird; the original distal carpals
coalesce with the proximal end of the metacarpals. These are reduced,
in all birds, to three, but traces of the fourth have been observed in
embryos. The first metacarpal is short and fuses throughout its length
with the second. This and the third are much longer and fuse together
at their upper and distal ends, leaving as a rule a space between the
shafts. The pollex and the third finger are as a rule reduced to one
phalanx each, while the index still has two. The first and second
fingers frequently carry a little claw. The greatest reduction of the
hand-skeleton is met with in _Dromaeus_ and in _Apteryx_, which retain
only the index finger. It is of importance for our understanding of
the position of the Ratitae in the system, that the wing-skeleton of
the ostrich and rhea is an exact repetition of that of typical flying
birds; the bones are much more slender, and the muscles are
considerably reduced in strength also to a lesser extent in numbers,
but the total length of the wing of an ostrich or a rhea is actually
and comparatively enormous. Starting with the kiwi and cassowary,
people have got into the habit of confounding flightless with wingless
conditions. It is absolutely certain that the wings of the Ratitae
bear the strongest testimony that they are the descendants of typical
flying birds.

The _pelvis_ (fig. 13), consisting of the sacrum (already described)
and the pelvic arch, namely ilium, ischium and pubis, it follows that
only birds and mammals possess a pelvis proper, whilst such is
entirely absent in the Amphibia and in reptiles with the exception of
some of the Dinosaurs. The ventral inner margin of the preacetabular
portion of the ilium is attached to the pre-sacral vertebrae, whilst
the inner and dorsal margin of the postacetabular portion is attached
to the primary sacral and the postsacral vertebrae. In rare cases the
right and left preacetabular blades fuse with each other above the
spinous processes. In front of the acetabulum a thick process of the
ilium descends to meet the pubis, and a similar process behind meets
the ischium. The acetabulum is completely surrounded by these three
bones, but its cup always retains an open foramen; from its posterior
rim arises the strong antitrochanter. The ischium and postacetabular
ilium originally enclose the ischiadic notch or _incisura ischiadica_.
This primitive condition occurs only in the Odontornithes (q.v.),
Ratitae and Tinami; in all others this notch becomes converted into a
_foramen ischiadicum_, through which pass the big stems of the
ischiadic nerves and most of the blood-vessels of the hind-limb. The
pubis consists of a short anterior portion (_spina pubica_ or
pectineal process, homologous with the prepubic process of Dinosaurs)
and the long and slender pubis proper (equivalent to the _processus
lateralis pubis_ of most reptiles). The shaft of the pubis runs
parallel with that of the ischium, with which it is connected by a
short ligamentous or bony bridge; this cuts off from the long
_incisura pubo-ischiadica_ a proximal portion, the _foramen
obturatum_, for the passage of the obturator nerve. Only in the
ostrich the distal ends of the pubes meet, forming a dagger-shaped
symphysis, which is curved forwards. The pectineal process is
variable; it may grow entirely from the pubis, or both pubis and ilium
partake of its formation, or lastly its pubic portion may be lost and
the process is entirely formed by the ilium. It is largest in the
Galli and some of the Cuculi, in others it is hardly indicated. It
served originally for the origin of the ambiens muscle (see _Muscular
System_ below); shifting or disappearance of this muscle, of course,
influences the process.

_The Hind Limb._--The femur often possesses a well visible pneumatic
foramen on the median side of the proximal end of its shaft. The inner
condyle, the intercondylar sulcus, and a portion only of its outer
condyle, articulate with corresponding facets of the tibia. The outer
condyle articulates mainly with the fibula. There is a patella,
intercalated in the tendon of the _femori-tibialis_ or _extensor
cruris_ muscle. In _Colymbus_ the patella is reduced to a small
ossicle, its function being taken by the greatly developed pyramidal
_processus tibialis anterior_; in _Podiceps_ and _Hesperornis_ the
patella itself is large and pyramidal. The distal half of the fibula
is very slender and normally does not reach the ankle-joint; it is
attached to the peroneal ridge of the tibia. On the anterior side of
the tibia, is the intercondylar sulcus, which is crossed by an oblique
bridge of tendon or bone, acting as a pulley for the tendon of the
_extensor digitorum communis_ muscle. The condyles of the tibia are in
reality not parts of this bone, but are the three proximal tarsalia
which fuse together and with the distal end of the tibia. The distal
tarsalia likewise fuse together, and then on to the upper ends of the
metatarsals; the _tarsale centrale_ remains sometimes as a separate
osseous nodule, buried in the inter-articular pad. Consequently the
ankle-joint of birds is absolutely cruro-tarsal and tarso-metatarsal,
i.e. intertarsal, an arrangement absolutely diagnostic of birds if it
did not also occur in some of the Dinosaurs. Of the metatarsals the
fifth occurs as an embryonic vestige near the joint; the first is
reduced to its distal portion, and is, with the hallux, shoved on to
the inner and posterior side of the foot, at least in the majority of
birds. The three middle metatarsals become fused together into a
cannon bone; the upper part of the third middle metatarsal projects
behind and forms the so-called hypotarsus, which in various ways,
characteristic of the different groups of birds (with one or more
sulci, grooved or perforated), acts as guiding pulley to the tendons
of the flexor muscles of the toes. Normally the four toes have two,
three, four and five phalanges respectively, but in _Cypselus_ the
number is reduced to three in the front toes. Reduction of the number
of toes (the fifth shows no traces whatever, not even in
_Archaeopteryx_) begins with the hallux, which is completely or partly
absent in many birds; the second toe is absent in _Struthio_ only. The
short feet of the penguins are quite plantigrade, in adaptation to
which habit the metatarsals lie in one plane and are incompletely
co-ossified, thus presenting a pseudo-primitive condition.

LITERATURE.--Only a mere fraction of the enormous literature dealing
with the skeleton of birds can here be mentioned.

M.E. Alix _Essai sur l'appareil locomoteur des oiseaux_ (Paris, 1874);
E. Blanchard, "Recherches sur les caracteres osteologiques des oiseaux
appliquees a la classification," _Ann. Sci. Nat. Ser._ iv., t. xi.; W.
Dames, "Ueber Brustbein Schulter- und Beckenguertel der Archaeopteryx,"
_Math. Naturw. Mitsh._, Berlin, vii., 1897, pp. 476-492; T.C. Eyton,
_Osteologia avium_ (London, 1858-1881), with many plates; C.
Gegenbaur, _Untersuch. z. vergl. Anat. d. Wirbelthiere, I. Carpus und
Tarsus, II. Schultergurtel_ (Leipzig, 1864-1865); P. Harting,
_L'Appareil episternal des oiseaux_ (Utrecht, 1864); T.H. Huxley, "On
the Classification of Birds and on the Taxonomic Value of the
Modifications of certain of the Cranial Bones..." _P.Z.S._, 1867; G.
Jaeger, "Das Wirbelkorpergelenk der Voegel," _Sitzb. K. Ak. Wiss._,
Wien, xxxiii., 1858; A. Johnson, "On the Development of the Pelvic
Girdle and Skeleton of the Hind-limb in the Chick," _Q.J.M.S._,
xxiii., 1883, pp. 399-411; K.F. Kessler, "Osteologie der Vogelfuesse,"
_Bull. Soc. Imp. Nat._, Moscow, xiv., 1841; B. Lindsay, "On the Avian
Sternum," _P.Z.S._, 1885; E. Mehnert, "Entwickelung des Ospelvis der
Voegel," _Morph. Jahrb._, xiii., 1877; A.B. Meyer, _Abbildungen van
Voegel-Skeletten_ (Dresden, 1879); St G. Mivart, "On the Axial Skeleton
of the Ostrich, Struthionidae, Pelecanidae," _Trans. Zool. Soc._
viii., 1874; x., 1877; E.S. Morse, "On the Carpus and Tarsus of
Birds," _Ann. Lyc. N.H._, New York, x., 1874; J.S. Parker,
"Observations on the Anatomy and Development of Apteryx," _Phil.
Trans._, 1890, pp. 1-110, 17 pls.; W. K. Parker, numerous papers in
_Trans. L.S., R.S._ and _Z.S._, e.g. "Osteology of Gallinaceous
Birds," _T.Z.S._, v., 1863; "Rhinochetus," _ibid._ vi.; "Skull of
Aegithognathous Birds," _ibid._, x., 1878; "Skull in the Ostrich
Tribe," _Phil. Trans._ vol. 156, 1866; "Skull of Common Fowl," _ibid._
vol. 159, 1870; "Skull of Picidae," _T. Linn. Soc._, 1875; "Monograph
on the Structure and Development of the Shoulder-girdle and Sternum,"
_Ray Soc._ London, 1868; W.P. Pycraft, "On the Morphology and
Phylogeny of the Palaeognathae (_Ratitae_ and _Crypturi_) and
Neognathae," _Trans. Zool. Soc._ xv., 1900, pp. 149-290, pis. 42-45;
id. "Some points in the morphology of the Palate of the Neognathae,"
_T. Linn. Soc._ 28, pp. 343-357, pls. 31-32; P. Suschkin, "Zur
Morphologie des Vogelskelets. I. Schaedel von Tinnunculus," _Mem. Soc.,
Moscow_, xvi., 1900, pp. 1-63, pls.

2. _Muscular System._

Of the muscles of the stem or axis, those of the neck and tail are well-developed and specialized, while those of the lower back are more or less reduced, or even completely degenerated owing to the rigidity of this region, brought about by the great antero-posterior extent of the pelvis.

The muscles of the limbs show a great amount of specialization, away from the fundamental reptilian and mammalian conditions. The muscles of the fore limbs are most aberrant, but at the same time more uniformly developed than those of the hinder extremities. The reasons are obvious. The whole wing is a unique modification, deeply affecting the skeletal, muscular and tegumentary structures, but fluttering, skimming, sailing, soaring are motions much more akin to one another than climbing and grasping, running, scratching, paddling and wading. The modifications of the hind-limbs are in fact many times greater (such as extremely long legs, with four, three or only two toes; very short legs, almost incapable of walking, with all four toes directed forwards, or two or one backwards, and two or more connected and therefore bound to act together, in various ways). Thus it has come to pass that the muscles of the hind limbs are, like their framework, more easily compared with those of reptiles and mammals than are the wings, whilst within the class of birds they show an enormous amount of variation in direct correlation with their manifold requirements. The only really aberrant modifications of the wing-muscles are found in the Ratitae, where they are, however, all easily explained by reduction, and in the penguins, where the wings are greatly specialized into blades for rowing with screw-like motions.

The wing of the bird is folded in a unique way, namely, the radius
parallel with the humerus, and the whole wrist and hand with their
ulnar side against the ulna; upper and forearm in a state of
supination, the hand in that of strong abduction. Dorsal and ventral
bending, even in the extended wing, is almost impossible. Consequently
only a few of the original extensor muscles have been preserved, but
these are much modified into very independent organs, notably the
_extensor metacarpi radialis longus_, the _ext. metac. ulnaris_ and
the two _radio_- and _ulnari-metacarpi_ muscles, all of which are
inserted upon the metacarpus by means of long tendons. The chief
muscular mass, arising from the sternum in the shape of a U, is the
_pectoralis_ muscle; its fibres converge into a strong tendon, which
is inserted upon the greater tubercle and upper crest of the humerus,
which it depresses and slightly rotates forwards during the
downstroke. This great muscle covers completely the
_supracoracoideus_, generally described as the second pectoral, or
_subclavius_ muscle, in reality homologous with the mammalian
_supraspinatus_ muscle. This arises mostly from the angle formed by
the keel with the body of the sternum, passes by a strong tendon
through the _foramen triosseum_, and is inserted upon the upper
tubercle of the humeral crest, which it rotates and abducts. The
extent of the origin of this muscle from the sternum, on which it
leaves converging, parallel or diverging impressions, is of some
taxonomic value.

From Newton's _Dictionary of Birds_, by permission of A. & C. Black.]

Much labour has been bestowed by A.H. Garrod and Max Fuerbringer upon
the investigation of the variations of the inserting tendons of the
patagial muscles (fig. 14), mainly from a taxonomic point of view. The
_propatagialis longus_ muscle is composed of slips from the deltoid,
pectoral, biceps and _cucullaris_ muscles. Its strong belly originates
near the shoulder joint from clavicle, coracoid and scapula. Its
elastic tendon runs directly to the carpus, forming thereby the outer
margin of the anterior patagium, or fold of skin between the upper and
forearm, which it serves to extend, together with the _propatagialis
brevis_ muscle. This runs down the anterior and outer side of the
upper arm, and is attached to the proximal tendon of the _extensor
metacarpi radialis longus_, a little below the outer condyle of the
humerus. In most birds the tendon is split into several portions, one
of which is often attached to the outer side of the ulna, below the
elbow joint, while others are in variable but characteristic ways
connected with similar slips of the _propatagialis longus_. The
posterior patagium, the fold between trunk and inner surface of the
upper arm, is stretched by the _metapatagialis_ muscle, which is
composed of slips from the _serratus, superficialis, latissimus dorsi_
and the _expansor secundariorum muscles_. This, the stretcher of the
cubital quills, is a very interesting muscle. Arising as a long tendon
from the sterno-scapular ligament, it passes the axilla by means of a
fibrous pulley, accompanies the axillary vessels and nerves along the
humerus, and is inserted by a few fleshy fibres on the base of the
last two or three cubital quills. Here, alone, at the distal portion
of the tendon, occur muscular fibres, but these are unstriped,
belonging to the category of cutaneous muscles. We have here the
interesting fact that a muscle (portion of the _triceps humeri_ of the
reptiles) has been reduced to a tendon, which in a secondary way has
become connected with cutaneous muscles, which, when strongly
developed, represent its belly.

The _flexor digitorum sublimis_ muscle arises fleshy from the long
elastic band which extends from the inner humeral condyle along the
ventral surface of the ulna to the ulnar carpal bone, over which the
tendon runs to insert itself on the radial anterior side of the first
phalanx of the second digit. Owing to the elasticity of the
humerocarpal band the wing remains closed without any special muscular
exertion, while, when the wing is extended, this band assists in
keeping it taut. The arm-muscles have been studied in an absolutely
exhaustive manner by Fuerbringer, who in his monumental work has
tabulated and then scrutinized the chief characters of fourteen
selected muscles. The results are as interesting from a morphological
point of view (showing the subtle and gradual modifications of these
organs in their various adaptations), as they are sparse in taxonomic
value, far less satisfactory than are those of the hind-limb. He was,
however, the first to show clearly that the Ratitae are the retrograde
descendants of flying ancestors, that the various groups of surviving
Ratitae are, as such, a polyphyletic group, and he has gone fully into
the interesting question of the development and subsequent loss of the
power of flight, a loss which has taken place not only in different
orders of birds but also at various geological periods, and is still
taking place. Very important are also the investigations which show
how, for instance in such fundamentally different groups as petrels
and gulls, similar bionomic conditions have produced step by step a
marvellously close convergence, not only in general appearance, but
even in many details of structure.

Of the muscles of the hind-limbs likewise only a few can be mentioned.
The _ambiens_ muscle, long and spindle-shaped, lying immediately
beneath the skin, extending from the pectineal process or ilio-pubic
spine to the knee, is the most median of the muscles of the thigh.
When typically developed its long tendon passes the knee-joint,
turning towards its outer side, and lastly, without being anywhere
attached to the knee, it forms one of the heads of the _flexor
perforatus digit_, ii. or iii. One of the functions of this peculiar
muscle (which is similarly developed in crocodiles, but absent, or not
differentiated from the ilio-tibial and ilio-femoral mass, in other
vertebrates) is that its contraction helps to close the second and
third toes. Too much has been made of this feature since Sir R. Owen
(_Cyclop. Anat. Phys_. i. p. 296, 1835), following G.A. Borelli (_De
motu animalium_, Rome, 1680), explained that birds are enabled to
grasp the twig on which they rest whilst sleeping, without having to
make any muscular exertion, because the weight of the body bends the
knee and ankle-joints, over both of which pass the tendons of this
compound muscle. There are many perching birds, e.g. all the Passeres,
which do not possess this muscle at all, whilst many of those which
have it fully developed, e.g. Anseres, can hardly be said to "perch."

Garrod went so far as to divide all the birds into _Homalogonatae_ and
_Anomalogonatae_, according to the presence or absence of the ambiens
muscle. This resulted in a failure. To appreciate this, it is
sufficient to enumerate the birds without the critical muscle:
_Passeriformes_ and _Coraciiformes_, without exception; _Ardeae_ and
_Podiceps_; lastly various genera of storks, pigeons, parrots, petrels
and auks. The loss has taken place, and still takes place,
independently in widely different groups. It follows, first, that the
absence of this muscle does not always indicate relationship; secondly
that we can derive birds that are without it from a group which still
possess it, but not vice versa. The absence of the ambiens muscle in
all owls, which apparently use their feet in the same way as the
_Accipitres_ (all of which possess it), indicates that owls are not
developed from the latter, but from a group which, like the other
_Coraciiformes_, had already lost their muscle.

Garrod further attributed much taxonomic value to the
_caudilio-femoralis_ muscle (fig. 15). This, when fully developed,
consists of two parts, but inserted by a single ribbon-like tendon
upon the hinder surface of the femur, near the end of its first third;
the caudal part, _femoro-caudalis_, expressed by Garrod by the symbol
_A_, arises from transverse processes of the tail; the iliac part
(_accessoro-femoro-caudal_ of Garrod, with the symbol _B_), arises
mostly from the outer surface of the postacetabular ilium. Of course
this double-headed condition is the more primitive, and as such exists
in most nidifugous birds, but in many of these, as well as in many
nidicolous birds, either the caudal or the iliac head is absent, and
in a very few (_Cancroma, Dicholophus, Steatornis_ and some
_Cathartes_) the whole muscle is absent. The _caud-ilio flexorius_
(_semitendinosus_ of most authors) arises from the transverse
processes of the tail, and from the distal half of the postacetabular
ilium, thence passing as a broad ribbon to the popliteal region, where
it splits into two portions. One of these, broad and fleshy, is
inserted upon the posterior surface of the distal third of the femur.
This portion, morphologically the original, was named the "accessory
semitendinosus" with the symbol _Y_; the other portion descends on the
hinder aspect of the leg and joins the fascia of the inner femoral
head of the _gastrocnemius_ muscle. In many birds the insertion is
shifted from the femur to the neck of the tibia, in which case the
"accessory head" is said to be absent, a condition expressed by Garrod
by the symbol _X_. By combining the four symbols _A, B, X, Y_,
according to their presence or absence, Garrod got a considerable
number of formulae, each of which was overruled, so to speak, by the
two categories of the presence or absence of the _ambiens_ muscle. It
needs hardly to be pointed out why such a purely mechanical scheme was
doomed to failure. Its author, with a considerable mathematical and
mechanical bias, reckoned entirely with the quantity, not with the
quality of his units, and relied almost implicitly upon his formulae.
It is, however, fair to state that his system was not built entirely
upon these muscular variations, but rather upon a more laborious
combination of anatomical characters, which were so selected that they
presumably could not stand in direct correlation with each other,
notably the oil-gland, caeca, carotids, nasal bones and above all, the
muscles of the thigh. He was, indeed, the first to show clearly the
relationship of the heron-like birds with the Steganopodes; of
stork-like birds with the American vultures; the great difference
between the latter and the other birds of prey; the connexion of the
gulls and auks with the plovers, and that of the sand-grouse with the
pigeons--discoveries expressed in the new terms of the orders
Ciconiiformes and Charadriiformes. These are instances, now well
understood, that almost every organic system, even when studied by
itself, may yield valuable indications as to the natural affinities of
the various groups of birds. That Garrod has so very much advanced the
classification of birds is ultimately due to his comprehensive
anatomical knowledge and general insight.

From Newton's _Dictionary of Birds._

A, Caudal. N, Sciatic nerve.
B, Iliac portion of caud-ilio-femoralis. Is.fm, Ischio-femoralis.
X, Caud-ilio-flexorius. Is.fl, Ischio-fibularis.
Y, "Accessory" portion of the same. Sart, Sartorius.
Pif, Pubischio-femoralis.]

To return to these thigh muscles. The most primitive combination,
ambiens and _A B X Y_, is the most common; next follows that of _A X
Y_, meaning the reduction of _B_, i.e. the iliac portion of the
_caud-ilio-femoralis_; _A B X_ and _B X Y_ are less common; _A X_ and
_X Y_ are rare and occur only in smaller groups, as in subfamilies or
genera; _B X_ occurs only in _Podiceps_. But the greatest reduction,
with only _A_ remaining, is characteristic of such a heterogeneous
assembly as Accipitres, Cypselidae. Trochilidae, Striges and Fregata.
This fact alone is sufficient proof that these conditions, or rather
reductions, have been acquired independently of the various groups. _A
B Y_, _A Y_, _A B_, _X Y_ and _B_ do not occur at all, some of them
for obvious reasons. Occasionally there is an instructive progressive
evolution expressed in these formula; for instance _Phaethon_, in
various other respects the lowest of the Steganopodes, has _A X Y_,
_Sula_ and _Phalacrocorax_ have _A X_, _Fregata_, the most specialized
of these birds, has arrived at the reduced formula _A_. Further, the
combinations _B X Y_ and _A X Y_ cannot be derived from each other,
but both directly from _A B X Y_ in two different directions. Keeping
this in mind, we may fairly conclude that the flamingo with _B X Y_
points to an ancestral condition _A B X Y_, which is still represented
by _Platalea_ and _Ibis_, whilst the other storks proper have taken a
different line, leading to _A X Y_.