MACKIE — ON FOSSIL BIRDS. 



19 



elucidation of the question) into the consideration of the structure, 

 not only of birds but of both mammals and reptiles also, the latter, 

 by the Pterodactyles, being intimately connected with the subject. 



"Amongst mammifers, the bats alone have the power of maintaining a 

 continued flight, by the rapid vibrations of membranous wings, which are 

 in fact but a modification of the arm and hand, so adapted as to enable 

 them to extend a thin canvas by which their bodies are drawn through 

 the air. To this end the fingers are greatly increased, in length as well as the 

 humerus and radius ; the scapula also is large, and lengthened in a slanting 

 direction at its lower portion for the better attachment of muscular power ; 

 and to afford greater support to the stroke of the wing ; the clavicles are 

 curved like the furcula of the bird, and perform the united functions of 

 the furcula and coracoids, preventing the compression of the chest, and 

 keeping the humerus in its place when acted upon by the pectoral muscles. 

 . . . But there is no deviation in this flying mammal from the plan nature 

 has laid down for the construction of mammifers ; the bat, because it flies, 

 is not a bird, no more than the bird that swims and cannot fly is a fish. . . . 

 The wing of the bat is in accordance with the same design that framed the 

 hand of man. . . . The bones of bats are excessively strong, hard, and 

 semitransparent. . . . The muscles of the fore arm are extremely insigni- 

 ficant when compared with those of birds ; their bones are harder in texture, 

 and only the very large bats possess Haversian tubes, even the largest 

 indigenous to this country are without them;* and the Pteropus, the 

 largest of the family, has only short and straight ones, and these not at all 

 numerous. ... In fig. 1, pi. vi. are shown the Haversian tubes and lacunse 

 of the Pteropus or flying fox, as it is popularly called ; the Haversian 

 tubes are more like long lacunse, and are in the humerus straight, uncon- 

 nected, and not numerous, but the lacunse are very abundant ; these be- 

 come more fusiform in the phalanges (fig. 2). . . . The canaliculi of the bat 

 are large for mammals. ... I conceive the reason of the paucity or absence 

 of these tubes (the Haversian) in the bats is to be found in the character 

 of their wings, and the habits of the animal. Bats require light but strong 

 bones, hardly at all flexible, for were they so the great length and slender- 

 ness would make the wing so pliable that it would be powerless in resist- 

 ing the air, and thereby enabling the animal to sustain itself in flight. Light- 

 ness is obtained by numerous lacunse, and numerous and comparatively 

 thick canaliculi, and no more elasticity is imparted to the bone than is 

 sufficient to preserve its structure, and obviate any fracture in flight. In 

 the bird, on the contrary, elastic resistance to muscular pressure is of 

 much importance ; and hence appears to have arisen that admirable ap- 

 plication and adjustment of the Haversian tubes so conspicuous in the 

 economy of birds. . . . Besides bats there are other mammif ers, such as the 

 Galeopithecus, flying Phalangers, etc., which . . . are enabled to perform 

 flights to some extent by the aid of a lateral extension of their skin, which 

 being spread out like a parachute sustains them for some time. ... In the 

 smaller flying Phalanger the Haversian tubes, especially in the tibia, are 

 large and numerous, the lacunse are very numerous, and the canaliculi 

 large ; every particle of unnecessary weight seems to have been abstracted 

 from their bones, that the light animal may float almost like a feather in 

 the air (fig. 3). . . . 



" Amongst recent reptiles, the Draco volans is enabled, by an extension of 



* In a footnote Mr. Dennis states that he has " found them in the jaw, a thick stem 

 with a few straight branches between the. fangs of the teeth." 



