112 THE DESCENT OF MAN. [Pakt I. 



Effects of the increased Use and Disuse of Parts. — 

 It is well known that use strengthens the muscles in the 

 individual, and complete disuse, or the destruction of the 

 proper nerve, weakens them. When the eye is destroyed 

 the optic 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 acting 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. 20 

 Different occupations habitually followed lead to changed 

 proportions in various parts of the body. Thus it was 

 clearly ascertained by the United States Commission 21 

 that the legs of the sailors employed in the late war were 

 longer by 0.217 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 sup- 

 porting weights. The girth of the neck and the depth of 

 the instep are greater, while the circumference of the 

 chest, waist, and hips, is less in sailors than in soldiers. 



Whether the several foregoing modifications would be- 

 come hereditary, if the same habits of life were followed 

 during many generations, is not known, but is probable. 

 Rengger 22 attributes the thin legs and thick arms of the 

 Payaguas Indians to successive generations having passed 



20 1 have given authorities for these several statements in my ' Varia- 

 tion of Animals under Domestication,' vol. ii. pp. 297-300. Dr. Jaeger, 

 " Ueber das Langenwachsthum dor Knochen," ' Jenaischen Zeitschrift,' 

 B. v. Heft i. 



21 'Investigations,' etc. By B. A. Gould, 1869, p. 288. 



2£ ' Siiugethiere von Paraguay,' 1830, s. 4. 



