The changes which we somewhat roughly and empirically group together as
the effects of "use and disuse" are of widely diverse character. Thus
bone, as the physiological fact, thickens under _alternations_ of
pressure (and the consequent increased flow of nourishment), but
atrophies under a steadily continued pressure; so that if the use of a
bone involved continuous pressure, the effect of such use would be a
partial or total absorption of that bone. Darwin shows that bone
lengthens as well as thickens from carrying a greater weight, while
tension (as seen in sailors' arms, which are used in pulling) appears to
have an equally marked effect in shortening bones (_Descent of Man_, p.
32). Thus different kinds of use may produce opposite results. The
cumulative inheritance of such effects would often be mischievous. The
limbs of the sloth and the prehensile tail of the spider monkey would
continually grow shorter, while the legs of the evolving elephant or
rhinoceros might lengthen to an undesirable extent. Such cumulative
tendencies of use-inheritance, if they exist, are obviously well kept
under by natural selection.
Although the ultimate effect of use is generally growth or enlargement
through increased flow of blood, the first effect usually is a loss of
substance, and a consequent diminution of size and strength. When the
loss exceeds the growth, use will diminish or deteriorate the part used,
while disuse would enlarge or perfect it. Teeth, claws, nails, skin,
hair, hoofs, feathers, &c., may thus be worn away faster than they can
renew themselves. But this wearing away usually stimulates the repairing
process, and so increases the rate of growth; that is, it will increase
the size produced, if not the size retained. Which effect of use does
use-inheritance transmit in such cases--the increased rate of growth, or
the dilapidation of the worn-out parts? We can hardly suppose that both
these effects of use will be inherited. Would shaving destroy the beard
in time or strengthen it? Will the continued shearing of sheep increase
or lessen the growth of wool? What will be the ultimate effect of
plucking geese's quills, and of the eider duck's abstraction of the down
from her breast? If the mutilated parts grow stronger or more
abundantly, why were the motmot's feathers alleged to be narrowed by the
inherited effects of ancestral nibbling?
The "use" or "work" or "function" of muscles, nerves, bones, teeth,
skin, tendon, glands, ducts, eyes, blood corpuscles, cilia, and the
other constituents of the organism, is as widely different as the
various parts are from each other, and the effects of their use or
disuse are equally varied and complicated.