124 THE WONDER OF LIFE 



breathing Vertebrates Reptiles, Birds and Mammals 

 the problem of flight has been solved, each time in a 

 different way. 



The power of taking ' soaring ' leaps has been acquired 

 many times over in the history of Vertebrates. R. S. 

 Lull gives ten cases Rhacophorus, Ptychozoon (a lizard 

 with a long fringed tail), Draco (a lizard with the skin 

 extended on greatly prolonged ribs), and seven Mammals, 

 Petauroides, Petaurus, Aerobates, Anomalurus, Pteromys, 

 Galeopithecus and Propithecus. Except in Petauroides, 

 there is in these swooping mammals a fold of skin along 

 the animal's flanks, which may be supplemented by 

 folds in front of the fore-limbs, between the hind limbs, 

 or along the tail. In the much-debated movements 

 of the Flying Fishes (Thoracopterus, Gigantopterus, 

 Exocoetus, and Dactylopterus), there is at most an 

 approximation to true flight. 



It is not surprising that many of the attempts to possess 

 the air should have proved quite unsuccessful, for man's 

 own experience of aviation has taught him that success 

 depends on numerous fine adjustments, and is not to be 

 attained except at great cost of life. In the case of birds 

 there is a remarkable correlation of numerous adaptations 

 the somewhat boat-like shape of the body, the ballasting 

 of the body with heavy organs below, the lightly built 

 skeleton with bones of the hollow girder type, the arrange- 

 ment by which the flying helps the breathing, the enormous 

 development of the pectoral muscles sometimes attaining to 

 half the whole weight of the bird, the turning of an arm into 

 a wing, the possession of feathers with inter-linked barbs, 

 the fusion of dorsal vertebrao to form a steady basis against 

 which the wings can work, and so on through a long list. 



