The effect of use is curiously illustrated in the form and figure of the giraffe. This animal, the largest of mammals, is found in the interior of Africa, where the ground is scorched and destitute of grass, and has to browse on the foliage of trees. From the continual stretching thus necessitated over a great space of time in all the individuals of the race, it has resulted that the fore legs have become longer than the hind legs, and that the neck has become so elongated that the giraffe, without standing on its hind legs, can raise its head to a height of nearly twenty feet. Observation of all animals will furnish similar examples. None, perhaps, is more striking than that of the kangaroo. This animal, which carries its young in an abdominal pouch, has acquired the habit of carrying itself upright upon its hind legs and tail, and of moving from place to place in a series of leaps, during which, in order not to hurt its little ones, it preserves its upright posture. Observe the result. (1) Its front limbs, which it uses very little, resting on them only in the instant during which it quits its erect posture, have never acquired a development in proportion to the other parts; they have remained thin, little, and weak. (2) The hind legs, almost continually in action, whether to bear the weight of the whole body or to execute its leaps, have, on the contrary, obtained a considerable development; they are very big and very strong. (3) Finally, the tail, which we observe to be actively employed, both to support the animal's weight and to execute its principal movements, has acquired at its base a thickness and a strength that are extremely remarkable. When the will determines an animal to a certain action, the organs concerned are forthwith stimulated by a flow of subtle fluids, which are the determining cause of organic changes and developments. And multiplied repetitions of such acts strengthen, extend, and even call into being the organs necessary to them. Now, every change in an organ which has been acquired by habitual use sufficient to originate it is reproduced in the offspring if it is common to both the individuals which have come together for the reproduction of their species. In the end, this change is propagated and passes to all the individuals which come after and are submitted to the same conditions, without its being necessary that they should acquire it in the original manner.