178 DEVELOPMENT AND HEREDITY. 



cannot be greatly changed without changing the 

 fundamental co-ordination of forces, and thus chang- 

 ing the other limb. Thus the development of the 

 leg is not the result of forces and stimuli acting 

 upon it alone, but it is also modified by forces act- 

 ing upon the arm, and vice versa. To this same 

 principle must we also attribute the exact (sym- 

 metrical) similarity between right and left limbs. 

 The shghtest deviation from the type of growth in 

 the right limb tends to appear equally in the left ; 

 if one hand is large or small, the other is the same, 

 and the feet correspond in relative size to the hands. 

 Little things only skin-deep, as moles, may be found 

 exactly corresponding on opposite sides, and even 

 the ridges of the skin on the finger-tips, which are 

 said to differ in every individual, resemble each 

 other in the right and left hands. That the devel- 

 opment of one arm influences the development of 

 the other is most strikingly shown in the nervous 

 and muscular co-ordinations and changes acquired 

 by the practice of writing. Generally, the right arm 

 alone has been trained to write. But if we take a 

 pencil in each hand, we can write with each hand 

 the same letters, except that the left hand, accord- 

 ing to its symmetrical relation to the right, produces 

 the letters in a symmetical reverse order. If a name 

 written thus with the left hand be held before a 



