The anatomical structure of man is closely similar to that of the
anthropoid apes--the gorilla, the orang, the chimpanzee, and the gibbon.
Bone for bone, muscle for muscle, blood-vessel for blood-vessel, nerve
for nerve, man and ape agree. As the conservative anatomist, Sir
Richard Owen, said, there is between them "an all-pervading similitude
of structure." Differences, of course, there are, but they are not
momentous except man's big brain, which may be three times as heavy as
that of a gorilla. The average human brain weighs about 48 ounces; the
gorilla brain does not exceed 20 ounces at its best. The capacity of the
human skull is never less than 55 cubic inches; in the orang and the
chimpanzee the figures are 26 and 27-1/2 respectively. We are not
suggesting that the most distinctive features of man are such as can be
measured and weighed, but it is important to notice that the main seat
of his mental powers is physically far ahead of that of the highest of
the anthropoid apes.
Man alone is thoroughly erect after his infancy is past; his head
weighted with the heavy brain does not droop forward as the ape's does;
with his erect attitude there is perhaps to be associated his more
highly developed vocal organs. Compared with an anthropoid ape, man has
a bigger and more upright forehead, a less protrusive face region,
smaller cheek-bones and eyebrow ridges, and more uniform teeth. He is
almost unique in having a chin. Man plants the sole of his foot flat on
the ground, his big toe is usually in a line with the other toes, and he
has a better heel than any monkey has. The change in the shape of the
head is to be thought of in connection with the enlargement of the
brain, and also in connection with the natural reduction of the muzzle
region when the hand was freed from being an organ of support and became
suited for grasping the food and conveying it to the mouth.
Everyone is familiar in man's clothing with traces of the past
persisting in the present, though their use has long since disappeared.
There are buttons on the back of the waist of the morning coat to which
the tails of the coat used to be fastened up, and there are buttons,
occasionally with buttonholes, at the wrist which were once useful in
turning up the sleeve. The same is true of man's body, which is a
veritable museum of relics. Some anatomists have made out a list of
over a hundred of these _vestigial_ structures, and though this number
is perhaps too high, there is no doubt that the list is long. In the
inner upper corner of the eye there is a minute tag--but larger in some
races than in others--which is the last dwindling relic of the third
eyelid, used in cleaning the front of the eye, which most mammals
possess in a large and well-developed form. It can be easily seen, for
instance, in ox and rabbit. In man and in monkeys it has become a
useless vestige, and the dwindling must be associated with the fact that
the upper eyelid is much more mobile in man and monkeys than in the
other mammals. The vestigial third eyelid in man is enough of itself to
prove his relationship with the mammals, but it is only one example out
of many. Some of these are discussed in the article dealing with the
human body, but we may mention the vestigial muscles going to the
ear-trumpet, man's dwindling counterpart of the skin-twitching muscle
which we see a horse use when he jerks a fly off his flanks, and the
short tail which in the seven-weeks-old human embryo is actually longer
than the leg. Without committing ourselves to a belief in the entire
uselessness of the vermiform appendix, which grows out as a blind alley
at the junction of the small intestine with the large, we are safe in
saying that it is a dwindling structure--the remains of a blind gut
which must have been capacious and useful in ancestral forms. In some
mammals, like the rabbit, the blind gut is the bulkiest structure in the
body, and bears the vermiform appendix at its far end. In man the
appendix alone is left, and it tells its tale. It is interesting to
notice that it is usually longer in the orang than in man, and that it
is very variable, as dwindling structures tend to be. One of the
unpleasant expressions of this variability is the liability to go wrong:
hence appendicitis. Now these vestigial structures are, as Darwin said,
like the unsounded, i.e. functionless, letters in words, such as the _o_
in "leopard," the _b_ in "doubt," the _g_ in "reign." They are of no
use, but they tell us something of the history of the words. So do man's
vestigial structures reveal his pedigree. They must have an historical
or evolutionary significance. No other interpretation is possible.
[Illustration: _Photo: New York Zoological Park._