116 THE DESCENT OF MAN. Part I . 



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 Ave treat of the different races of man- 

 kind. 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 evi- 

 dence 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 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 pro- 

 portions 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 sol- 

 diers, though the sailors were on an average shorter 

 men ; whilst their arms were shorter by 1*09 of an inch, 

 and therefore out of proportion shorter in relation to 



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

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

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

 B. v. Heft i. 



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



