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
supposed to indicate their gradual disappearance through want of room in
a diminishing jaw. But a note on Tasmanian skulls in the _Catalogue of
the College of Surgeons_ (p. 199) shows that this lateness and
irregularity have been common among Tasmanians as well as among
civilized races, so that the change can hardly be attributed to the
effects of disuse under civilization.