oot have answered the purpose
of walking quite as well? But as survivals their presence is fully
accounted for, since they are indispensable to many of the lower
animals. Question may also be made of the utility of the large number of
bones in the wrist and heel of man. Equal flexibility of the joint could
certainly have been obtained with a smaller number of bones. It is only
when these are traced back to their probable origin in the walking
organs of the fish ancestor of the batrachians that their presence
becomes explainable. They are apparently survivals of a very ancient
structure, originated for swimming, and adapted to walking.

As regards the wrist of man, a curious prediction that a certain bone
found in some of the lower animals, the _os centrale_, would be found in
man has been made and verified, it being discovered as a very small
rudiment in the human embryo. The tail, so common a feature in the lower
animals, but absent from the higher apes and from man, has not vanished
without leaving its traces. In the human embryo it is plainly indicated;
and while it vanishes in man beyond the embryo stage, it is simply
hidden beneath the skin, where its vertebrae are still apparent, usually
three, sometimes four or five, in number. In addition to this, the
muscles which move the tail have left traces of their presence, which
not infrequently develop into true muscles.

In the human embryo, indeed, we find ourselves in the midst of highly
significant indications of man's origin. The body of man passes in its
early development through a series of stages, in each of which it
resembles the mature or the embryo state of certain animals lower in the
stage of existence. It begins its existence as a simple cell, analogous
in form to the amoeba, one of the lowest living creatures, and later
assumes the gastrula form supposed to have been that of the earliest
many-celled animals. From this state it progresses by successive stages,
each of which has some relation in form to a lower class.

The most significant of these is that in which the embryo is closely
assimilated to the fish, by the possession of gill slits. There are four
of these openings in the neck of the human foetus, and they are at times
so persistent that children have been born with them still open, so that
fluids taken in at the mouth could trickle out at the neck, the opening
being sufficient to admit a thin probe.[2] These slits are utilized in
the developin