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, “Über das Langenwachsthum der Knochen,” ‘Jenäischen 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.