, probably owe their singular
conformation to analogous circumstances.

"The result of habits is as remarkable in the carnivorous mammals as
it is in the herbivorous, but it presents effects of another kind.

"Indeed, those of these mammals which are habituated, as their race,
both to climb as well as to scratch or dig in the ground, or to tear
open and kill other animals for food, have been obliged to use the
digits of their feet; moreover, this habit has favored the
separation of their digits, and has formed the claws with which they
are armed.

"But among the carnivores there are some which are obliged to run in
order to overtake their prey; moreover, since these need and
consequently have the habit of daily tearing with their claws and
burying them deeply in the body of another animal, to seize and then
to tear the flesh, and have been enabled by their repeated efforts
to procure for these claws a size and curvature which would greatly
interfere in walking or running on stony soil, it has resulted in
this case that the animal has been obliged to make other efforts to
draw back these too salient and curved claws which would impede it,
and hence there has resulted the gradual formation of those special
sheaths in which the cats, tigers, lions, etc., withdraw their claws
when not in action.

"Thus the efforts in any direction whatever, maintained for a long
time or made habitually by certain parts of a living body to satisfy
necessities called out by nature or by circumstances, develop these
parts and make them acquire dimensions and a shape which they never
would have attained if these efforts had not become the habitual
action of the animals which have exercised them. The observations
made on all the animals known will everywhere furnish examples.

"Can any of them be more striking than that which the _kangaroo_
offers us? This animal, which carries its young in its abdominal
pouch, has adopted the habit of holding itself erect, standing only
on its hind feet and tail, and only changing its position by a
series of leaps, in which it preserves its erect attitude so as not
to injure its young.

"Let us see the result:

"1. Its fore legs, of which it makes little use, and on which it
rests only during the instant when it leaves its erect attitude,
have never reached a development proportionate to that of the other
parts, and have rema