62 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



In the process of adaptation to flight the thumb (i.), 

 and the little finger (v.) first lost their nails and 

 became useless. Next, the metacarpal and the 

 proximal phalange of the thumb became much 

 smaller, and united with the bones of the wrist, 

 while the distal phalange degenerated to a mere 

 thread. Fresh evidences of degeneration presented 

 themselves as the specialization became more ad- 

 vanced. In Ptera- 

 nodon, for instance, 

 the metacarpals of 

 the clawed digits 

 were considerably 

 reduced in size, the 

 arms having almost 

 ceased to exercise 

 any function but 

 that of flight. There 

 was a special pro- 

 vision for the support 



Q f these great willgS, 



the development of which was only equalled among 

 vertebrates by that of the pectoral fins of the 

 skate. Whereas the legs were upheld by the pelvis, 

 which was in its turn supported by the sacrum, the 

 arms of Pteranodon were attached to the middle of 

 the shoulder blade, the latter being supported by 

 the vertebral column an absolutely unique con- 

 dition. This kind of pectoral sacrum implied the 

 fusion of a number of vertebrae, and involved a 



FIG. 31. Pteropus (a bat). 



Ill, IV,' V, digil 



of Vertebrated Animals.) 



Skeleton of fore- 



