Here we perceive a difficulty which will equally trouble those who
affirm use-inheritance and those who deny. Broadly speaking, the
adaptive effects ascribed to use-inheritance coincide with the effects
of natural selection. The individual adaptability (as shown in the
thickening of skin, fur, muscle, &c., under the stimulus of friction,
cold, use, &c.) is identical in kind and direction with the racial
adaptability under natural selection. Consequently the alleged
inheritance of the advantageous effects of use and disuse cannot readily
be distinguished from the similarly beneficial effects of natural
selection. The indisputable fact that natural selection imitates or
simulates the beneficial effects ascribed to use-inheritance may be the
chief source and explanation of a belief which may prove to be
thoroughly fallacious. A similar simulation of course occurs under
domestication, where natural selection is partly replaced by artificial
selection of the best adapted and therefore most flourishing animals,
while in disused parts panmixia or the comparative cessation of
selection will aid or replace "economy of growth" in causing
diminution.[45]