Aim and Method of Comparative Anatomy 



369 



Fig. 293. Convergence. Adaptation for swimming in fish, reptile, and mammal. 

 (Top) Shark, a fish. {Center) Ichthyosaur, an extinct reptile. {Bottom) Porpoise, a 

 modern mammal. (Courtesy, American Musum of Natural History, New York.) 



to a single organ instead of involving the whole body. Stiff, sharp 

 spines, such as are produced by the skin of our common porcupine, 

 occur also on the European hedgehog and on the "spiny anteater" of 

 the Australian region — animals genetically very far apart (Fig. 294). 

 The "flying membrane," a broad fold of skin extending along the side 

 of the body from foreleg to hindleg, is similarly developed in flying 

 squirrels (rodents) and in flying phalangers (Australian marsupials), 

 and reaches its extreme differentiation in bats (Fig. 547). The East 

 Indian "flying lemur" (Galeopithecus: affinities uncertain) has flying 

 membranes very much like those of the squirrel (Fig. 593). The wings 

 of the pterodactyls, extinct flying reptiles, were similar to those of 



