THE DESCENT OB ORIGIN OF MAN 53 



better food and greater comfort do inHuence stature. But 

 the preceding statements sliow liow difficult it is to arrive 

 at any precise result. Dr. Beddoe has lately proved that, 

 with the inhabitants of Britain, residence in towns and cer- 

 tain occupations have a deteriorating influence on height; 

 and he infers that the result is to a certain extent inherited, 

 as is likewise the case in the United States. Dr. Beddoe 

 further believes that wherever a "race attains its maximum 

 of physical development, it rises highest in energy and 

 moral vigor." " 



Whether external conditions produce any other direct 

 effect on man is not known. It might have been expected 

 that differences of climate would have had a marked influ- 

 ence, inasmuch as the lungs and kidneys are brought into 

 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 dur- 

 ing many ages. But this subject will be more properly dis- 

 cussed when we treat of the different races of mankind. 

 With our domestic animals there are grounds for believ- 

 ing 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 kidney 



" "Memoirs, Authropolog. Soc," vol. iii., 1861-69, pp. 661, 565, 56'7. 

 *> Dr. Brakenridge, "Theory of Diathesis," "Medical Times," June 19, and 

 July 17, 1869. 



