"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 quote a multitude of examples of the effects of use and disuse upon our own organs, effects which, if the use and disuse were constant for many generations, would become much more marked.