FIG. 16.--The Flying Lemur _Galeopithecus volans_, Shaw. In the flying lemur the membranous expansion (_a b_) is more extensive than in the Flying Dragon (fig. 15). It is supported by the neck, back, and tail, and by the anterior and posterior extremities. The flying lemur takes enormous leaps; its membranous tunic all but enabling it to fly. The Bat, _Phyllorhina gracilis_ (fig. 17), flies with a very slight increase of surface. The surface exposed by the bat exceeds that displayed by many insects and birds. The wings of the bat are deeply concave, and so resemble the wings of beetles and heavy-bodied short-winged birds. The bones of the arm (_r_), forearm (_d_), and hand (_n, n, n_) of the bat (fig. 17) support the anterior or thick margin and the extremity of the wing, and may not inaptly be compared to the nervures in corresponding positions in the wing of the beetle.--_Original._] [Illustration: FIG. 17.--The Bat (_Phyllorhina gracilis_, Peters). Here the travelling-surfaces (_r d e f_, _a n n n_) are enormously increased as compared with that of the land and water animals generally. Compare with figures from 10 to 14, p. 34. _r_ Arm of bat; _d_ forearm of bat; _e f_, _n n n_ hand of bat.--_Original._] Although no lizard is at present known to fly, there can be little doubt that the extinct Pterodactyles (which, according to Professor Huxley, are intermediate between the lizards and crocodiles) were possessed of this power. The bat is interesting as being the only mammal at present endowed with wings sufficiently large to enable it to fly.[20] It affords an extreme example of modification for a special purpose,--its attenuated body, dwarfed posterior, and greatly elongated anterior extremities, with their enormous fingers and outspreading membranes, completely unfitting it for terrestrial progression. It is instructive as showing that flight may be attained, without the aid of hollow bones and air-sacs, by purely muscular efforts, and by the mere diminution and increase of a continuous membrane. [20] The Vampire Bat of the Island of Bonin, according to Dr. Buckland, can also swim; and this authority was of opinion that the Pterodactyle enjoyed similar advantages.--Eng. Cycl. vol. iv. p. 495. As the flying lizard, flying lemur, and bat (figs. 15, 16, and 17, pp. 35 and 36), connect terrestrial progression with aërial progression, so the auk, penguin (fig. 46, p. 91), and flying-fish (fig. 51, p. 98), connect progression in the water with progression in the air. The travelling surfaces of these anomalous creatures run the movements peculiar to the three highways of nature into each other, and bridge over, as it were, the gaps which naturally exist between locomotion on the land, in the water, and in the air. PROGRESSION ON THE LAND.