detailed correspondence, without
there being any instances to the contrary, if we pay due regard to
vestigial characters. The entire corporeal structure of man is an exact
anatomical copy of that which we find in the ape.

My object, then, here is to limit attention to those features of our
corporeal structure which, having become useless on account of our
change in attitude and habits, are in process of becoming obsolete, and
therefore occur as mere vestigial records of a former state of things.
For example, throughout the vertebrated series, from fish to mammals,
there occurs in the inner corner of the eye a semi-transparent eye-lid,
which is called the nictitating membrane. The object of this structure
is to sweep rapidly, every now and then, over the external surface of
the eye, apparently in order to keep the surface clean. But although the
membrane occurs in all classes of the sub-kingdom, it is more prevalent
in some than in others--e.g. in birds than in mammals. Even, however,
where it does not occur of a size and mobility to be of any use, it is
usually represented, in animals above fishes, by a functionless
rudiment, as here depicted in the case of man. (Fig. 10.)

[Illustration: FIG. 10.--Illustrations of the nictitating membrane
in the various animals named drawn from nature. The letter N
indicates the membrane in each case. In man it is called the _plica
semilunaris_, and is represented in the two lower drawings under
this name. In the case of the shark (_Galeus_) the muscular
mechanism is shown as dissected.]

Now the organization of man presents so many vestigial structures thus
referring to various stages of his long ancestral history, that it would
be tedious so much as to enumerate them. Therefore I will yet further
limit the list of vestigial structures to be given as examples, by not
only restricting these to cases which occur in our own organization; but
of them I shall mention only such as refer us to the very last stage of
our ancestral history--viz. structures which have become obsolescent
since the time when our distinctively human branch of the family tree
diverged from that of our immediate forefathers, the Quadrumana.

(1) _Muscles of the external ear._--These, which are of large size and
functional use in quadrupeds, we retain in a dwindled and useless
condition (Fig. 11). This is likewise the case in anthropoid apes; but
in not a few other Quadrumana (e.g. baboons