THE LIVING MACHINE BUILDING FACTORS. 167? 



fingers and bones of the arm were still distinct, 

 and yet the whole formed a true wing. Thus we 

 see that the common point of origin which is 

 suggested by the likenesses between an arm and 

 a wing is no mere imaginary 

 one, for the fossil record has 



FIG. 47. 



FIG. 48. 



FIG. 49. 



FIG. 47. The arm of a monkey, a prehensile appendage. 



FIG. 48. The arm of a bird, a flying appendage. In life covered with 



feathers. 

 FIG. 49. The arm of an ancient half-bird, half-reptile animal. In life 



covered with feathers and serving as a wing. 



shown us the path leading to that point of 

 origin. The whole tells us further that nature's 

 method of producing a grasping or flying organ 

 was here, not to build a new organ, but to take 

 one that had hitherto been used for other pur- 

 poses, and by slow changes modify its form 

 and function until it was adapted to new 

 duties. 



