how difficult it is to arrive at any precise result. Dr. Beddoe has
lately proved that, with the inhabitants of Britain, residence in towns
and certain 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 vigour. (19. Memoirs,
Anthropological Society, vol. iii. 1867-69, pp. 561, 565, 567.)
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 influence, inasmuch as the lungs and kidneys
are brought into activity under a low temperature, and the liver and
skin under a high one. (20. Dr. Brakenridge, Theory of Diathesis,
Medical Times, June 19 and July 17, 1869.) 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 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 A 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. I have given
authorities for these several statements in my Variation of Animals
and Plants under Domestication, vol. ii. pp. 297-300. Dr. Jaeger,
Uber das Langenwachsthum der Knochen, Jenaischen Zeitschrift, B. v.
Heft. i.) Different occupations, habitually followed, lead to changed
proportions in various parts of the body. Thus it was ascertained by
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