ANIMALS WHICH FLY
The nature of a reptile is now sufficiently intelligible for something
to be said concerning flight, and structures by means of which some
animals lift themselves in the air. It is not without interest to
remember that, from the earliest periods in human records,
representations have been made of animals which were furnished with
wings, yet walked upon four feet, and in their typical aspect have the
head shaped like that of a bird. They are commonly named Dragons.
FLYING DRAGONS
[Illustration: FIG. 3 From _The Battle between Bel and the Dragon_]
The effigy of the dragon survives to the present day in the figure over
which St. George triumphs, on the reverse of the British sovereign. In
the luxuriant imaginations of ancient Eastern peoples, dating back to
prehistoric ages, perhaps 5000 B.C., the dragons present an astonishing
constancy of form. In after-times they underwent a curious evolution, as
the conception of Babylon and Egypt is traced through Assyria to Greece.
The Wings, which had been associated at first with the fore limb of the
typical dragon, become characteristic of the Lion, and of the poet's
winged Horse, and finally of the Human figure itself, carved on the
great columns of the Greek temples of Ephesus. These flying animals are
historically descendants of the same common stock with the dragons of
China and Japan, which still preserve the aspect of reptiles. Their
interest is chiefly in evidence of a latent spirit of evolution in days
too remote for its meaning to be now understood, which has carried the
winged forms higher and ever higher in grade of organisation, till their
wings ceased to be associated with feelings of terror. The Hebrew
cherubim are regarded by H. E. Ryle, Bishop of Exeter, as probably
Dragons, and the figure of the conventional angel is the human form of
the Dragon.
[Illustration: FIG. 4. FIGURE FROM THE TEMPLE OF EPHESUS]
ORGANS OF FLIGHT
Turning from this reference to the realm of mythology to existing
nature, the power of flight is popularly associated with all the chief
types of vertebrate animals--fishes, frogs, lizards, birds, and mammals.
Many of the animals ill deserve the name of flyers, and most are
exceptions to different conditions of existence which control their
kindred, but it is convenient to examine for a little the nature of the
structures by which this movement in the air, which is not always
flight, is made possible. Certain fishes, like the lung-fish Ceratodus,
of Queensland, and the mud-fish Lepidosiren, are capable of leaving the
water and living on land, and for a time breathe air. But neither these
fishes nor Periophthalmus, which runs with rapid movement of its fins
and carries the body more or less out of water, or the climbing perch,
Anabas, carried out of water over the country by Indian jugglers, ever
put on the slightest approach to wings.
FLYING FISHES
[Illustration: FIG. 5. THE FLYING FISH EXOCOETUS
With the fins extended moving through the air]
The flight of fishes is a kind of parachute support not unlike that by
which a folded paper is made to travel in the air. It is chiefly seen in
the numerous species of a genus Exocoetus, allied to the gar-pike
(Belone), which is common in tropical seas, and usually from a foot to
eighteen inches long. They emerge from the water, and for a time support
themselves in the air by means of the greatly developed breast fins,
which sometimes extend backward to the tail fin. Although these fins
appear to correspond to the fore limbs of other animals, they may not be
moved at the will of the fish like the wing of a bird. When the flying
fishes are seen in shoals in the vicinity of ships, those fins remain
extended, so that the fish is said sometimes to travel 200 yards at a
speed of fifteen miles an hour, rising twenty feet or more above the
surface of the sea, travelling in a straight line, though sometimes
influenced by the wind. Here the organ, which is at once a fin and a
wing, consists of a number of thin long rods, or rays, which are
connected by membrane, and vary in length to form an outline not unlike
the wing of a bird which tapers to a point. The interest of these
animals is chiefly in the fact that flight is separated from the
condition of having lungs with which it is associated in birds, for
although the flying fish has an air bladder, there is no duct to connect
it with the throat.
FLYING FROGS
[Illustration: FIG. 6. THE FLYING FROG (RHACOPHORUS)
The membranes of the foot and hand extend between the metatarsal and
metacarpal bones, as well as the bones of the digits.]
Among amphibians the organs of flight are also of a parachute kind, but
of a different nature. They are seen in certain frogs which frequent
trees, and are limited to membranes which extend between the diverging
digits of the hand and foot, forming webs as fully developed as in the
foot of a swimming bird. As these frogs leap, the membranes are expanded
and help to support the weight of the body, so that the animal descends
more easily as it moves from branch to branch. There is no evidence that
the bones of the digits ever became elongated like the fin rays of the
flying fish or the wing bones of a Bat; but the web suggests the basis
of such a wing, and the possibilities under which wings may first
originate, by elongation of the bones of a webbed hand like that of a
Flying Frog.
FLYING LIZARDS
[Illustration: FIG. 7. THE FLYING DRAGON, DRACO
Forming a parachute by means of the extended ribs]
The Reptilia in their several orders are remarkable for absence of any
modification of the arms which might suggest a capacity for acquiring
wings, as being latent in their organisation. Crocodiles, Tortoises, and
Serpents are alike of the earth, and not of the air. But among Lizards
there are small groups of animals in which a limited capacity for
movement through the air is developed. It is best known in the family of
small lizards named Dragons, represented typically by the species _Draco
volans_ found in the Oriental region of the East Indies and Malay
Archipelago.
The organ of flight is produced in an unexpected way, by means of the
ribs instead of the limbs. The ribs extend outward as far as the arms
can stretch, and the first five or six are prolonged beyond the body so
as to spread a fold of skin on each side between the arm and the leg.
The membrane admits of some movement with the ribs. This arrangement
forms a parachute, which enables the animal to move rapidly among
branches of trees, extending the structure at will, so that it is used
with rapidity too quick to be followed by the eye, as it leaps through
considerable distances.
A less singular aid to movement in the air is found in some of the
lizards termed Geckos. The so-called Flying Gecko (_Platydactylus
homalocephalus_) has a fringe unconnected with ribs, which extends
laterally on the sides of the body and tail, as well as at the back and
front of the fore and hind limbs, and between the digits, where the web
is sometimes almost as well developed as among Tree Frogs. This is
essentially a lateral horizontal frill, extending round the body. Its
chief interest is in the circumstance that it includes a membrane which
extends between the wrist bones and the shoulder on the front of the
arm. That is the only part of the fringe which represents the wing
membrane of a bird. The fossil flying reptiles have not only that
membrane, but the lateral membranes at the sides of the body and behind
the arms.
Other lizards have the skin developed in the direction of the
circumference of the body. In the Australian Chlamydosaurus it forms an
immense frill round the neck like a mediaeval collar. But though such an
adornment might break a fall, it could not be regarded as an organ of
flight.
FLYING BIRDS
[Illustration: FIG. 8. POSITION OF BIRDS IN FLIGHT]
The wings of birds, when they are developed so as to minister to flight,
are all made upon one plan; but as examples of the variation which the
organs contributing to make the fore limb manifest, I may instance the
short swimming limb of the Penguin, the practically useless rudiment of
a wing found in the Ostrich or Kiwi, and the fully developed wing of the
Pigeon. The wings of birds obtain an extensive surface to support the
animal by muscular movements of three modifications of structure. First,
the bones of the fore limb are so shaped that they cannot, in existing
birds, be applied to the ground for support and be used like the limbs
of quadrupeds, and are therefore folded up at the sides of the body,
and carried in an unused or useless state so long as the animal hops on
the ground or walks, balancing its weight on the hind legs. Secondly,
there are two small folds of skin, less conspicuous than those on the
arms of Geckos; one is between the wrist bones and the shoulder, and the
smaller hinder membrane is between the upper arm and the body. These
membranous expansions are insignificant, and would in themselves be
inadequate to support the body or materially assist its movements.
Thirdly, the bird develops appendages to the skin which are familiarly
known as feathers, and the large feathers which make the wing are
attached to the skin covering the lower arm bone named the ulna, and the
other bones which represent the wrist and hand. The area and form of the
bird's wing are due to individual appendages to the skin, which are
unknown in any other group of animals. Between the extended wing of the
Albatross, measuring eleven feet in spread, and the condition in the
Kiwi of New Zealand, in which the wing is vanishing, there is every
possible variation in size and form. As a rule, the larger the animal
the smaller is the wing area. The problem of the origin of the bird's
wing is not to be explained by study of existing animals; for the rowing
organ of the Penguin, which in itself would never suggest flight,
becomes an organ of flight in other birds by the growth upon it of
suitable feathers. Anyone who has seen the birds named Divers feeding
under water, swimming rapidly with their wings, might never suspect that
they were also organs of aerial flight. The Ostrich is even more
interesting, for it has not developed flight, and still retains at the
extremities of two of the digits the slender claws of a limb which was
originally no wing at all, but the support of a four-footed animal (Fig.
46, p. 130).
FLYING MAMMALS
Flight is also developed among mammals. The Insectivora include several
interesting examples of animals which are capable of a certain motion
through the air. In the tropical forests of the Malay Archipelago are
animals known as Flying Squirrels, Flying Opossums, Flying Lemurs,
Flying Foxes, in which the skin extends outward laterally from the sides
of the body so as to connect the fore limbs with the hind limbs, and is
also prolonged backward from the hind limbs to the tail. The four digits
are never elongated; the bones of the fore limb are neither longer nor
larger than those of the hind limb, and the foot terminates in five
little claws as in other four-footed animals. This condition is adapted
for the arboreal life which those animals live, leaping from branch to
branch, feeding on fruits and leaves, and in some cases upon insects.
These mammals may be compared with the Flying Geckos among reptiles in
their parachute-like support by extension of the skin, which gives them
one of the conditions of support which contribute to constitute flight.
[Illustration: FIG. 9. FLYING SQUIRREL (PTEROMYS)]
_Bats._--One entire order of mammals--the Bats--not only possess true
wings, but are capable of flight which is sustained, and in some cases
powerful. The wings are clothed with short hair like the rest of the
body, and thus the instrument of flight is unlike that of a bird. The
flight of a Bat differs from that of all other animals in being
dependent upon a modification of the bones of the fore limb, which,
without interfering with the animal's movements as a quadruped, secures
an extension of the wing which is not inferior in area to that which the
bird obtains by elongation of the bones of the arm and fore-arm and its
feathers. The distinctive peculiarity of the Bat's wing is in the
circumstance that four of the digits of the hand have their bones
prolonged to a length which is often equal to the combined length of the
arm and fore-arm. The bones of the digits diverge like the ribs of an
umbrella, and between them is the wing membrane, which extends from the
sides of the body outward, unites the fore limb with the hind limb, and
is prolonged down the tail as in the Flying Foxes. Bats have a small
membrane in front of the bones of the arm and fore-arm stretching
between the shoulder and the wrist, which corresponds with the wing
membrane of a bird; but the remainder of the membranes in Bats' wings
are absent in birds, because their function is performed by feathers
which give the wing its area. The elongated digits of the Bat's wing are
folded together and carried at the sides of the body as though they were
a few quill pens attached to its wrist, where the one digit, which is
applied to the ground in walking, terminates in a claw.
[Illustration: FIG. 10 NEW ZEALAND BAT FLYING. BARBASTELLE WALKING]
The organs which support animals in the air are thus seen to be more or
less dissimilar in each of the great groups of animals. They fall into
three chief types: first, the parachute; secondly, the wing due to the
feathers appended to the skin; and thirdly, the wing formed of membrane,
supported by enormous elongation of the small bones of the back of the
hand and fingers. The two types of true wings are limited to birds and
bats; and no living reptile approximates to developing such an organ of
flight as a wing. Judged, therefore, by the method of comparing the
anatomical structures of one animal with another, which is termed
"comparative anatomy," the existence of flying reptiles might be
pronounced impossible. But in the light which the revelations of geology
afford, our convictions become tempered with modesty; and we learn that
with Nature nothing is impossible in development of animal structure.