sideration relative to this question.

"Light does not penetrate everywhere; consequently animals which
habitually live in situations where it does not penetrate lack the
occasion of exercising the organs of sight, if nature has provided
them with them. Moreover, the animals which make part of the plan of
organization in which _eyes_ are necessarily present, have
originally had them. However, since we find them among those which
are deprived of the use of this organ, and which have only vestiges
concealed and covered over, it should be evident that the
impoverishment and even the disappearance of these organs are the
result of a constant lack of exercise.

"What proves it is that the organ of _hearing_ is never in this
condition, and that we always find it in the animals when the nature
of their organization should require its existence; the reason is as
follows.

"The _cause of sound_, that which, moved by the shock or the
vibrations of bodies, transmits to the organ of hearing the
impression which it receives, penetrates everywhere, traverses all
the media, and even the mass of the densest bodies: from this it
results that every animal which makes a part of a plan of
organization to which _hearing_ is essential, has always occasion to
exercise this organ in whatever situation it lives. So, among the
_vertebrate animals_ we see none deprived of their organs of
hearing; but in the groups below them, when the same organs are once
wanting, we do not again find them.

"It is not so with the organ of sight, for we see this organ
disappear, reappear, and again disappear, in proportion to the
possibility or impossibility of the animal's exercising it.

"In the _acephalous molluscs_, the great development of the mantle
of these molluscs has rendered their eyes and even their head
entirely useless. These organs, also forming a part of a plan of
organization which should comprise them, have disappeared and
atrophied from constant lack of use.

"Finally, it is a part of the plan of organization of _reptiles_, as
in other vertebrate animals, to have four legs appended to their
skeleton. The serpents should consequently have four, though they do
not form the lowest order of reptiles, and are not so near the
fishes as the batrachians (the frogs, the salamanders, etc.).

"However, the serpents having taken up the habit of gliding along
the ground,