The next example, the reduced biting muscles, &c., of lap-dogs is also
unsatisfactory as a proof of the inheritance of the effects of disuse;
for the change can readily be accounted for without the introduction of
such a factor. The previous natural selection of strong jaws and teeth
and muscles is reversed. The conscious or unconscious selection of
lap-dogs with the least tendency to bite would easily bring about a
general enfeeblement of the whole biting apparatus--weakness of the
parts concerned favouring harmlessness. Mr. Spencer maintains that the
dwindling of the parts concerned in clenching the jaw is certainly not
due to artificial selection because the modifications offer no
appreciable external signs. Surely hard biting is sufficiently
appreciable by the person bitten without any visual admeasurement of the
masseter muscles or the zygomatic arches. Disuse during lifetime would
also cause some amount of degeneracy; and I am not sure that Mr. Spencer
is right in _entirely_ excluding economy of nutrition from the problem.
Breeders would not over-feed these dogs; and the puppies that grew most
rapidly would usually be favoured.