derable length (the
ant-eater, green wood-pecker); when it is obliged to seize anything
with this same organ, then its tongue will divide and become forked.
That of the humming-birds, which seize with their tongue, and that
of the lizard and serpents, which use it to feel and examine objects
in front of them, are proofs of what I advocate.

"Wants, always occasioned by circumstances, and followed by
sustained efforts to satisfy them, are not limited in results, in
modifying--that is to say, in increasing or diminishing--the extent
and the faculties of organs; but they also come to displace these
same organs when certain of these wants become a necessity.

"The fishes which habitually swim in large bodies of water, having
need of seeing laterally, have, in fact, their eyes placed on the
sides of the head. Their bodies, more or less flattened according to
the _species_, have their sides perpendicular to the plane of the
water, and their eyes are placed in such a way that there is an eye
on each flattened side. But those fishes whose habits place them
under the necessity of constantly approaching the shores, and
especially the shelving banks or where the slope is slight, have
been forced to swim on their flattened faces, so as to be able to
approach nearer the edge of the water. In this situation, receiving
more light from above than from beneath, and having a special need
of being always attentive to what is going on above them, this need
has forced one of their eyes to undergo a kind of displacement, and
to assume the very singular situation which is familiar to us in the
_soles_, _turbots_, _dabs_, etc. (_Pleuronectes_ and _Achirus_). The
situation of these eyes is asymmetrical, because this results from
an incomplete change. Now, this change is entirely completed in the
rays, where the transverse flattening of the body is entirely
horizontal, as also the head. Also the eyes of the rays, both
situated on the upper side, have become symmetrical.

"The serpents which glide along the surface of the ground are
obliged chiefly to see elevated objects, or what are above their
eyes. This necessity has brought an influence to bear on the
situation of the organs of vision in these animals; and, in fact,
they have the eyes placed in the lateral and upper parts of the
head, so as to easily perceive what is above or at their sides; but
they only see for a