which appear to be the earliest mammals that
confined themselves to a life upon the ground, have but two hooves,
while the horse has only one.[308]

"Some herbivorous animals, especially among the ruminants, have been
incessantly preyed upon by carnivorous animals, against which their only
refuge is in flight. Necessity has therefore developed the light and
active limbs of antelopes, gazelles, &c. Ruminants, only using their
jaws to graze with, have but little power in them, and therefore
generally fight with their heads. The males fight frequently with one
another, and their desires prompt an access of fluids to the parts of
their heads with which they fight; thus the horns and bosses have arisen
with which the heads of most of these animals are armed.[309] The
giraffe owes its long neck to its continued habit of browsing upon
trees, whence also the great length of its fore legs as compared with
its hinder ones. Carnivorous animals, in like manner, have had their
organs modified in correlation with their desires and habits. Some
climb, some scratch in order to burrow in the earth, some tear their
prey; they therefore have need of toes, and we find their toes separated
and armed with claws. Some of them are great hunters, and also plunge
their claws deeply into the bodies of their victims, trying to tear out
the part on which they have seized; this habit has developed a size and
curvature of claw which would impede them greatly in travelling over
stony ground; they have therefore been obliged to make efforts to draw
back their too projecting claws, and so, little by little, has arisen
the peculiar sheath into which cats, tigers, lions, &c., withdraw their
claws when they no longer wish to use them.[310]

"We see then that the long-sustained and habitual exercise of any part
of a living organism, in consequence of the necessities engendered by
its environment, develops such part, and gives it a form which it would
never have attained if the exercise had not become an habitual action.
All known animals furnish us with examples of this.[311] If anyone
maintains that the especially powerful development of any organ has had
nothing to do with its habitual use--that use has added nothing, and
disuse detracted nothing from its efficiency, but that the organ has
always been as we now see it from the creation of the particular species
onwards--I would ask why cannot our domesticated ducks fly like wild
ducks? I would also quot