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 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 evi- 
dence on this head in the case of man. 
Effects of the increased JJse 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. AVhen 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, 
“ Ueberjlas Langenwachsthum der Knochen,” ‘ Jenaischen Zeitsclirift/ 
B. v. Heft i. 
21 ‘ Investigations/ &c. By B. A. Gould, 1869, p. 288. 
