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.

CROWDED TEETH.

The too closely-packed teeth in the "decreasing" jaws of modern men (p.
13)[5] are also suggestive of other causes than use and disuse. Why is
there not simultaneous variation in teeth and jaws, if disuse is the
governing factor? Are we to suppose that the size of the human teeth is
maintained by use at the same time that the jaws are being diminished by
disuse? Mr. Spencer acknowledges that the crowding of bull-dogs' and
lap-dogs' teeth is caused by the artificial selection of shortened jaws.
If a similar change is really occurring in man, could it not be
similarly explained by some factor, such as sexual selection, which
might affect the outward appearance at the cost of less obvious defects
or inconveniences?

Mr. Spencer points to the decay of modern teeth as a sign or result of
their being overcrowded through the diminution of the jaw by disuse.[6]
But the teeth which are the most frequently overcrowded are the lower
incisors. The upper incisors are less overcrowded, being commonly
pressed outwards by the lower arc of teeth fitting inside them in
biting. The lower incisors are correspondingly pressed inwards and
closer together. Yet the upper incisors decay--or at least are
extracted--about twenty times as frequently as the closely packed lower
incisors.[7] Surely this must indicate that the cause of decay is not
overcrowding.

The lateness and irregularity of the wisdom teeth are sometimes
s