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. 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 the United States Commission (22. 'Investigations,' etc.,
by B.A. Gould, 1869, p. 288.) 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. Rengger (23. 'Saugethiere von Paraguay,' 1830, s. 4.)
attributes the thin legs and thick arms of the Payaguas Indians to