nt, let us prove it by the citation of the leading
known facts on which it is based.

"The vertebrate animals, whose plan of organization is in all nearly
the same, although they offer much diversity in their parts, have
jaws armed with _teeth_; moreover, those among them which
circumstances have placed in the habit of swallowing their food
without previous _mastication_ are exposed to the result that their
teeth become undeveloped. These teeth, then, either remain concealed
between the bony edges of the jaws, without appearing above, or even
their gums are found to have been atrophied.

"In the baleen whales, which have been supposed to be completely
deprived of teeth, M. Geoffroy has found them concealed in the jaws
of the _foetus_ of this animal. This professor has also found in
the birds the groove where the teeth should be situated; but they
are no longer to be seen there.

"In the class even of mammals, which comprises the most perfect
animals, and chiefly those in which the vertebrate plan of
organization is most perfectly carried out, not only the baleen has
no usable teeth, but the ant-eater (_Myrmecophaga_) is also in the
same condition, whose habit of not masticating its food has been for
a long time established and preserved in its race.

"The presence of eyes in the head is a characteristic of a great
number of different animals, and becomes an essential part of the
plan of organization of vertebrates.

"Nevertheless the mole, which owing to its habits makes very little
use of vision, has only very small eyes, which are scarcely visible,
since they exercise these organs to a very slight extent.

"The _Aspalax_ of Olivier (_Voyage en Egypte et en Perse_, ii.
pl. 28 f. 2), which lives under ground like the mole, and which
probably exposes itself still less than that animal to the light of
day, has totally lost the power of sight; also it possesses only
vestiges of the organ of which it is the seat; and yet these
vestiges are wholly concealed under the skin and other parts which
cover them, and do not permit the least access to the light.

"The _Proteus_, an aquatic reptile allied to the salamander in its
structure, and which lives in the dark subterranean waters of deep
caves, has, like the _Aspalax_, only vestiges of the organs of
sight--vestiges which are covered and concealed in the same manner.

"We turn to a decisive con