Mr. Spencer verified this by comparing English jaws with Australian and
Negro jaws at the College of Surgeons.[2] He maintains that the
diminution of the jaw in civilized races can _only_ have been brought
about by inheritance of the effects of lessened use. But if English jaws
are lighter and thinner than those of Australians and Negroes, so too is
the rest of the skull. As the diminution in the weight and thickness of
the walls of the cranium cannot well be ascribed to disuse, it must be
attributed to some other cause; and this cause may have affected the jaw
also. Cessation of the process by which natural selection[3] favoured
strong thick bones during ages of brutal violence might bring about a
change in this direction. Lightness of structure, facilitating agility
and being economical of material, would also be favoured by natural
selection so far as strength was not too seriously diminished.
Sexual selection powerfully affects the human face, and so must affect
the jaws--as is shown by the differences between male and female jaws,
and by the relative lightness and smallness of the latter, especially in
the higher races. Human preference, both sexual and social, would tend
to eliminate huge jaws and ferocious teeth when these were no longer
needed as weapons of war or organs of prehension, &c. We can hardly
assume that the lower half of the face is specially exempt from the
influence of natural and sexual selection; and the effects of these
undoubted factors of evolution must be fully considered before we are
entitled to call in the aid of a factor whose existence is questioned.
After allowing for lost teeth and the consequent alveolar absorption,
and for a reduction proportional to that shown in the rest of the skull,
the difference in average weight in fifty European and fourteen
Australian male jaws at the College of Surgeons turned out to be less
than a fifth of an ounce, or about 5 per cent. This slight reduction may
be much more than accounted for by such causes as disuse in the
individual, human preference setting back the teeth, and partial
transference of the much more marked diminution seen in female jaws.
There is apparently no room for accumulated _inherited_ effects of
ancestral disuse. The number of jaws is small, indeed; but weighing them
is at least more decisive than Mr. Spencer's mere inspection.
The differences between Anglo-Saxon male jaws and Australian and
Tasmanian jaws are most easily explained as effects of human preference
and natural selection. We can hardly suppose that disuse would maintain
or develop the projecting chin, increase its perpendicular height till
the jaw is deepest and strongest at its extremity, evolve a side flange,
and enlarge the upper jaw-bone to form part of a more prominent nose,
while drawing back the savagely obtrusive teeth and lips to a more
pleasing and subdued position of retirement and of humanized beauty. If
human preference and natural selection caused some of these differences,
why are they incompetent to effect changes in the direction of a
diminution of the jaw or teeth? And if use and disuse are the sole
modifying agents in the case of the human jaw, why should men have any
more chin than a gorilla or a dog?
The excessive weight of the West African jaws at the College of Surgeons
is partly _against_ Mr. Spencer's contention, unless he assumes that
Guinea Negroes use their jaws far more than the Australians, a
supposition which seems extremely improbable. The heavier skull and
narrower molar teeth point however to other factors than increased use.
The striking variability of the human jaw is strongly opposed to the
idea of its being under the direct and dominant control of so uniform a
cause as ancestral use and disuse. Mr. Spencer regards a variation of 1
oz. as a large one, but I found that the English jaws in the College of
Surgeons varied from 1.9 oz. to 4.3 oz. (or 5 oz. if lost teeth were
allowed for); Australian jaws varied from 2 oz. to 4.5 oz. (with _no_
lost teeth to allow for); while in Negro jaws the maximum rose to over
5-1/2 oz.[4] In spite of disuse some European jaws were twice as heavy
as the lightest Australian jaw, either absolutely or (in some cases)
relatively to the cranium. The uniformity of change relied upon by Mr.
Spencer is scarcely borne out by the facts so far as male jaws are
concerned. The great reduction in the weight of _female_ jaws _and
skulls_ evidently points to sexual selection and to panmixia under male
protection.
I think, on the whole, we must conclude that the human jaws do not
afford satisfactory proof of the inheritance of the effects of use and
disuse, inasmuch as the differences in their weight and shape and size
can be more reasonably and consistently accounted for as the result of
less disputable causes.