32 The Descent of Man. Part i 



temperature, and the liver and skin under a high one. 20 It was 

 formerly thought that the colour 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 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 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 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 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. 21 

 Different occupations, habitually followed, lead to changed 

 proportions in various parts of the body. Thus it was ascertained 

 by the United States Commission 22 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 ; whilst 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, whilst 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 followed during many 

 generations, is not known, but it is probable. Eengger 23 attri 

 butes the thin legs and thick arms of the Payaguas Indians to 



20 Dr. Brakenridge, ' Theory of Dr. Jaeger, " Ueber das I.angen- 

 Diathesis,' ' Medical Times,' June 19 wachsthum der Knochen," ' Jena- 

 and July 17, 1869. ischen Zeitschrift,' B. v. Heft i 



21 I have given authorities for 22 ' Investigations,' &c. By a. a, 

 these several statements in my Gould, 18G9, p. 288. 

 'Variation of Animals under Do- 23 ' S'augethiere yon Paraguay 

 mesticat'on,' vol. ii. pp. 297-300. 1830, s. 4. 



