36 THE DESCENT OF MAN. 



activity under a low temperature, and the liver and skin 

 under a high one. * It was formerly thought that the color 

 of the skin and the character of the hair were determined 

 by light or heat; and although it can hardly be denied that 

 some effect is thus produced, almost all observers now agree 

 that the effect has been very small, even after exposure 

 during many ages. But this subject will be more properly 

 discussed when we treat of the different races of mankind. 

 With our domestic animals there are grounds for believing 

 that cold and damp directly affect the growth of the hair; 

 but I have not met with any evidence on this head in the 

 case of man. 



Effects of the Increased Use and Disuse of Parts. It is 

 well known that use strengthens the muscles in the individ- 

 ual, 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 kid- 

 ney ceases to act from disease, the other increases in size, 

 and does double work. Bones increase not only in thick- 

 ness, but in length, from carrying a greater weight, f Dif- 

 ferent occupations, habitually followed, lead to changed 

 proportions in various parts of the body. Thus it was as- 

 certained by the United States Commission^ 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 heiglit. This shortness of 

 the arms is apparently due to their greater use, and is an 

 unexpected result: but sailors chiefly use their arms in pull- 

 ing, and not in supporting weights. With sailors, the girth 

 of the neck and the depth of the instep are greater, while 



*Dr. Brakenridge, "Theory of Diathesis," ''Medical Times," 

 June 19 and July 17, 1869. 



f I have given authorities for these several statements in my 

 "Variation of Animals under Domestication," vol. ii, pp. 297-800 

 Dr. Jaeger, " Ueber das Langenwachsthum der Knochen," " Jenais 

 chen Zeitschrift," B. v, Heft. i. 



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



