46 THE THEORY OF EVOLUTION 



the primitive, five-toed and mobile anterior limb into 

 structures which are purely locomotive in function. 



It is evident that the same elements are present 

 in the human hand and arm as in the fore leg of the 

 horse and ox, but in each case characteristically 

 modified to serve different ends. Man's hand, for- 

 tunately for us, is really a very primitive and un- 

 differentiated structure and can be put to a great 

 many different uses. Had it been highly specialized 

 for a single purpose, human progress and civilization 

 would have been impossible, for these have always 

 depended upon the co-ordination of hand, eye and 

 brain. In the horse everything has been sacrificed 

 to speed, making the animal a "cursorial machine," 

 and, to this end, the functional digits have been 

 reduced from five to one, the proportionate lengths 

 of the various segments of the limb adjusted to the 

 best advantage and the feet raised from the ground, 

 so that the horse, as also the ox, walks upon the very 

 tips of the toes. What is ordinarily called the "knee ' 

 of a horse is the exact homologue of the human wrist, 

 while the elbow joint is concealed in the muscles of 

 the shoulder. 



Again, the flipper of a whale is, to all external ap- 

 pearances, totally different from the human arm 

 and hand, as from the fore leg of the horse, but dis- 

 section shows that the plan of structure is the same, 

 identical parts being greatly modified in form and 

 function in accordance with the exclusively marine 

 life of the animal. Except at the shoulder, there is 



