Читать книгу Dragons of the Air: An Account of Extinct Flying Reptiles - H. G. Seeley - Страница 20

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.

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.

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.