HOMOLOGY. 71 



two animals somewhat closely related are compared, 

 let us say the dog compared with man, it is found 

 that they are built upon exactly the same plan, and 

 can be compared with each other bone for bone and 

 muscle for muscle. The more thorough be the 

 comparison the greater appears the likeness, for the 

 very abnormalities strengthen it. Muscles occa- 

 sionally found in one are universally present in the 

 other ; and so with special protuberances of bones, 

 with nerves, and with almost every other feature ; 

 everything shows that there is a complete homology 

 between the bodies of different vertebrates, and it is 

 the duty of the anatomist to discover these homolo- 

 gies. 



What is the exact answer which the theory of 

 evolution will give to the question of the real 

 explanation of this homology? A man's arm and a 

 bird's wing are organs with totally different func- 

 tions and very different in shape ; yet in spite of this 

 difference they are fundamentally alike in their 

 muscles and bones. The theory of evolution will 

 say that this likeness is due to the fact that both 

 man and bird have descended from a common 

 ancestor, living long ago, which possessed an 

 appendage, neither an arm nor a leg, but containing 

 the fundamental structure of both, /. e., the features 

 which they possess in common. The descendants 

 of this ancestor in the course of many generations 

 became modified in two directions and their appen- 

 dages were correspondingly modified. On the one 

 hand the descendants became flying animals and 

 the anterior limb was developed into a wing, retain- 



