DESCENT OF MAN FROM SOME LOWER FORM. 157 



nerve often becomes atrophied. When an artery is tied, 

 the lateral channels increase not only in diameter, but in 

 the thickness and strength of their coats. When one 

 kidney ceases to act from disease, the other increases in 

 size, and does double work. Bones increase not only in 

 thickness, but in length, from carrying a greater weight. 

 Different occupations, habitually followed, lead to changed 

 proportions in various parts of the body. Thus it was 

 ascertained by the United States commission that the 

 legs of the sailors employed in the late war were longer 

 by - 217 of an inch than those of the soldiers, though the 

 sailors were on an average shorter men ; while their arms 

 were shorter by 1*09 of an inch, and therefore, out of 

 proportion, shorter in relation to their lesser height. This 

 shortness of the arms is apparently due to their greater 

 use, and is an unexpected result ; but sailors chiefly use 

 their arms in pulling, and not in supporting weights. 

 With sailors, the girth of the neck and the depth of the 

 instep are greater, while the circumference of the chest, 

 waist, and hips is less, than in soldiers. 



Whether the several foregoing modifications would 

 become hereditary, if the same habits of life were fol- 

 lowed during many generations, is not known, but it is 

 probable. 



In infants, long before birth, the skin on 



the soles of the feet is thicker than on any 



other part of the body ; and it can hardly be doubted that 



this is due to the inherited effects of pressure during a 



a long series of generations. 



It is familiar to every one that watchmakers and en- 

 gravers are liable to be short-sighted, while men living 

 much out-of-doors, and especially savages, are generally 

 long-sighted. Short-sight and long-sight certainly tend 



