their tongues; serpents and lizards use their tongues to
touch and reconnoitre objects in front of them, hence their tongues have
come to be forked.

"Need--always occasioned by the circumstances in which an animal is
placed, and followed by sustained efforts at gratification--can not only
modify an organ, that is to say, augment or reduce it, but can change
its position when the case requires its removal.[305]

"Ocean fishes have occasion to see what is on either side of them, and
have their eyes accordingly placed on either side their head. Some
fishes, however, have their abode near coasts on submarine banks and
inclinations, and are thus forced to flatten themselves as much as
possible in order to get as near as they can to the shore. In this
situation they receive more light from above than from below, and find
it necessary to pay attention to whatever happens to be above them; this
need has involved the displacement of their eyes, which now take the
remarkable position which we observe in the case of soles, turbots,
plaice, &c. The transfer of position is not even yet complete in the
case of these fishes, and the eyes are not, therefore, symmetrically
placed; but they are so with the skate, whose head and whole body are
equally disposed on either side a longitudinal section. Hence the eyes
of this fish are placed symmetrically upon the uppermost side.[306]

"The eyes of serpents are placed on the sides and upper portions of the
head, so that they can easily see what is on one side of them or above
them; but they can only see very little in front of them, and supplement
this deficiency of power with their tongue, which is very long and
supple, and is in many kinds so divided that it can touch more than one
object at a time; the habit of reconnoitring objects in front of them
with their tongues has even led to their being able to pass it through
the end of their nostrils without being obliged to open their jaws.[307]

"Herbivorous mammals, such as the elephant, rhinoceros, ox, buffalo,
horse, &c., owe their great size to their habit of daily distending
themselves with food and taking comparatively little exercise. They
employ their feet for standing, walking, or running, but not for
climbing trees. Hence the thick horn which covers their toes. These toes
have become useless to them, and are now in many cases rudimentary only.
Some pachyderms have five toes covered with horn; some four, some
three. The ruminants,