proportionate to the bulk of their body; this is heavy, their wings

small and frail, and so the flight they use is like a cargo boat

attempting to make its voyage with oars; now the frailty both of the

actual wings and of the outgrowths upon them contributes in a

measure to the flight described. Among birds, the peacock's tail is at

one time useless because of its size, at another because it is shed.

But birds are in general at the opposite pole to flying insects as

regards their feathers, but especially the swiftest flyers among them.

(These are the birds with curved talons, for swiftness of wing is

useful to their mode of life.) The rest of their bodily structure is

in harmony with their peculiar movement, the small head, the slight

neck, the strong and acute breastbone (acute like the prow of a

clipper-built vessel, so as to be well-girt, and strong by dint of its

mass of flesh), in order to be able to push away the air that beats

against it, and that easily and without exhaustion. The hind-quarters,

too, are light and taper again, in order to conform to the movement of

the front and not by their breadth to suck the air.

11



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,