TRANSFORMATION OF ORGANS OF ANIMALS 63 



consequent degeneration of the muscles which in 

 earlier Pterosaurians caused the movement of one 

 vertebra upon another. 



Two phases may thus be observed in the evolu- 

 tion of the Pterosaurians : first, Pterodactylus and 

 Rhamplwrynclius, and next Pteranodon, while de- 

 generation attended the modifications of both. 



(c) Bats. Bats, like the Pterosaurians, fly by 

 means of a membrane, only that the membrane, 

 instead of principally extending from the little 

 finger to the body, is equally developed between 

 all the fingers. 



Let us consider what degeneration is involved by 

 this modification. 



Firstly, all the digits have lost their nails, except- 

 ing the thumb and the index finger in fruit-eating 

 bats, and the thumb alone in insect-eating bats, 

 while some of the phalanges of the digits are 

 missing, the usual number being two instead of 

 three. Further, instead of the ulna and radius 

 being equally developed in the fore-arm, the ulna 

 is greatly reduced in size. 



5. Adaptation to arboreal life. We have seen 

 that, as a rule, the first evidence of adaptation to 

 arboreal life is the opposability of the great toe. 

 This modification is accompanied by the degeneration 

 of the nail, for, instead of consisting of a claw cover- 

 ing the entire extremity of the last phalange, it 

 becomes a small nail covering only the upper side. 



Another modification is exhibited at a slightly 



