other embryonic survivals yet to be
pointed out. As regards the seeming function of the thyroid, it may be
said that the surviving relic of an ancient functional organ is quite
capable of varying in structure and taking upon itself a new function,
of minor value, which in its absence would be left undone or be
performed by some of the other organs.

A highly interesting example of this exists in the swim-bladder of the
fish, which there is good reason to believe is a survival of an ancient
structure used for quite a different purpose. It was originally
developed, in the opinion of the writer,[1] as an air-breathing organ,
in a very ancient semi-amphibious class of fishes, from which the
existing bony fishes have descended. When the latter resumed the
gill-breathing habit, this organ lost its original function, and its
subsequent history is a curious and significant one. In some modern
fishes it has quite disappeared. In others it exists as a minute and
useless remnant, no larger than a pea. In many it has been converted
into the swim-bladder, and in this form serves a useful purpose, but
varies very greatly in shape and size. Finally, in a few instances, it
retains some measure of its probably original function of air-breathing.
It is a fact of much significance, that those fishes without a
swim-bladder do not seem to be at any disadvantage from its absence, but
are able to make their way vertically through the water quite as well as
those which possess this organ. The presumption, therefore, is that it
is of little utility to the fish, and that its employment for this
purpose is a mere resultant of its survival and character. Such an organ
could never have been evolved as an aid in swimming, since its shrinkage
to a useless rudiment in some cases and its complete extinction in
others show that this function is in no sense a necessary one. It is
there and has lost its old use, and is, in some cases, adapted to
another purpose; that is all that can be said.

Man is the one hairless mammal,--or hairless except on a few parts of
his body. Yet the whole body is covered with a thin growth of hair,
useless for any purpose of protection, and only explainable as a
survival from the mammalian covering. The occasional considerable
development of the hair is an indication pointing to such an origin.
This applies not only to individuals, but to tribes or races, as in the
instances of the Ainos of Japan and some of the Pygmies of