for observing can see for himself, this has been accounted for from the fact that the thighs are flexed against the abdomen during the hitter part of intra-uterine life. But from analogy with other young creatures, such as those already mentioned and young birds, we find that the pre-natal position has little or no influence in decreeing the habitual attitude of the limbs after birth, and it seems to me more logical and reasonable to trace this also to a prior state of evolutionary development.
Man is, when standing erect, the only animal that has the thigh in a line with the axis of the vertebral column, and among his nearest congeners in the animal world the flexed state of the femoral articulation is natural and constant. As we go down the scale the angle between the thighs and trunk diminishes, until it reaches the right angle characteristic of most quadrupeds. I speak here of the attitude adopted when the animal is at rest upon its legs, for during sleep there is in many cases a curious reversion to the position occupied in embryonic life. Thus we see that a bird roosting with its head "under its wing," and the legs drawn up close to the body, offers a decided resemblance to the chick in the egg.
I have noticed that young children, when old enough to shift their limbs, very seldom sleep in any but the curled-up position; and that as often as not, when unhampered by clothing or other artificial restraints, they sleep in the same attitude as do many quadrupeds, viz., with the abdomen downward and the limbs flexed beneath them. I am told that negro mothers and nurses in the West Indies invariably lay their charges down to sleep on their stomachs, and that this custom is also common in various parts of the world. Adult man is, I believe, the only animal who ever elects to sleep upon his back. Some of the lower savages seem to sleep comfortably on occasion in a crouching position with the head bent down upon the knees, just as all the common tribes of monkeys do. Among the quadrumana it is not until we come to the platform-building anthropoid types that we find a recumbent position habitually taken during sleep. The young orangs and chimpanzees that they have had at the Zoölogical Gardens slept with the body semi-prone and with the limbs, or all except one arm, which was used as a pillow, curled under them. This is exactly the position voluntarily adopted by eighty per cent of children between ten and twenty months old which I have had opportunities of watching. I was told by the attendants at the Zoölogical Gardens that no ape will sleep flat on his back, as adult man often does.
It would be very interesting to get exact observations as to the habits of all the lower tribes of men with regard to sleeping, for it is a point upon which a great deal would seem to depend, if.
