so much then for these questions. but why an animal that is to stand erect must necessarily be not only a biped, but must also have the superior parts of the body lighter, and those that lie under these heavier, is plain. only if situated like this could it possibly carry itself easily. and so man, the only erect animal, has legs longer and stouter relatively to the upper parts of his body than any other animal with legs. what we observe in children also is evidence of this. children cannot walk erect because they are always dwarf-like, the upper parts of their bodies being longer and stouter than the lower. with advancing years the lower increase disproportionately, until the children get their appropriate size, and then and not till then they succeed in walking erect. birds are hunchbacked yet stand on two legs because their weight is set back, after the principle of horses fashioned in bronze with their forelegs prancing. but their being bipeds and able to stand is above all due to their having the hip-bone shaped like a thigh, and so large that it looks as if they had two thighs, one in the leg before the knee-joint, the other joining his part to the fundament. really this is not a thigh but a hip, and if it were not so large the bird could not be a biped. as in a man or a quadruped, the thigh and the rest of the leg would be attached immediately to quite a small hip; consequently the whole body would be tilted forward. as it is, however, the hip is long and extends right along to the middle of the belly, so that the legs are attached at that point and carry as supports the whole frame. it is also evident from these considerations that a bird cannot possibly be erect in the sense in which man is. for as it holds its body now the wings are naturally useful to it, but if it were erect they would be as useless as the wings of cupids we see in pictures. it must have been clear as soon as we spoke that the form of no human nor any similar being permits of wings; not only because it would, though sanguineous, be moved at more than four points, but also because to have wings would be useless to it when moving naturally. and nature makes nothing contrary to her own nature.