CHAPTER XI
Physical Qualities of the Chimpanzee—His Social Habits—Mental Characteristics
Physically considered, the chimpanzee very closely resembles man, but there are certain points in which he differs both from man and from other apes. We may notice a few of these points. The model of the ear of the chimpanzee closely resembles that of man, but the organ is larger in size and thinner in proportion. It is very sensitive to sound, but dull to touch. The surface is not well provided with nerves. He cannot erect his ear, as most animals do, by the use of the muscles at the base; but, like the human ear, the muscles are useless, and in this respect the ear is fixed and helpless.
The hand of the chimpanzee is long and narrow. The finger bones are larger, in proportion to their size, than those of the human hand. One thing peculiar to the hand of the chimpanzee is that the tendons inside of the hand (those called the flexors), which are designed to close the finders, are shorter than the line of the bones. On this account the fingers of the ape are always held in a curve. He cannot straighten them. This is probably due to the habit of climbing, in which he indulges to such a great extent. He also indulges in the practice of hanging suspended by the hands. In making his way through the bush he often swings himself by the arms from bough to bough. Sometimes he suspends himself by one arm, while he uses the other to pluck and eat fruit. This characteristic is transmitted to the young, and is found in the first stages of infancy. The thumb is not truly opposable, but is inclined to close towards the palm of the hand. It is of little use to him. His nails are thick, dark in color, and not quite so flat as those of man.
The great toe, instead of being in line with the others, projects at an angle from the side of the foot, something after the manner of the human thumb. The foot itself is quite flexible and has great prehensile power. In climbing, and in many other ways, it is used as a hand. The tendons in the sole of the foot are equal in length to the line of the bones, and the digits of the foot can be straightened; but from the habitual use of them in climbing, the ape is predisposed to close the digits, wherefore the foot is naturally inclined to curve into an arch, especially in the line of the first and second digits.
His habit of walking is peculiar. The greater part of the weight is borne upon the legs. The sole of the foot is placed almost flat on the ground, but the pressure is greatest along the outer edge, in the line of the last digit. This is easily noticed where he walks over plastic ground. In the act of walking he always uses the hands, but he does not place the palms on the ground. He uses the backs of the fingers instead. Sometimes only the first joints or phalanges, resting upon the nails, are placed on the ground. At other times the first and second joints are used. I have seen one specimen that, when walking, employed the backs of all his fingers, from the knuckles to the nails. The integument on these parts is not callous, like that of the palm. The color pigment is distributed the same as on other exposed parts of the body. These facts show that the weight of the body is not borne on the fore limbs, as it is in the case of a true quadruped, but indicate that the hand is only used to balance the body while in the act of walking and to shift the weight from foot to foot. The weight is, therefore, not equally distributed between the hands and the feet, and the animal cannot truly be said to be a quadruped in habit.
His waddling gait is caused by his short legs, stooping habit, and heavy body. All animals having stout bodies and short legs are predisposed to a waddling motion, which is due to the wide angle between the weight and the changing center of gravity. This motion is more conspicuous in bipeds than in quadrupeds, because the base supporting the weight is reduced to a single point.
The chimpanzee is neither a true quadruped nor a true biped, but combines the habits of both. It appears to be a transition state from the former to the latter. Vestiges of this mixed habit are still to be found in man. In the act of walking his arms alternate in motion with his legs. This suggests the idea that he may have had, at some time, a similar habit of locomotion. Such a fact does not necessarily show that he was ever an ape, but it does point to the belief that he has once occupied a horizon in nature like that now occupied by the ape, and that having emerged from it, he still retains traces of the habit. This peculiarity is still more easily observed in children than in adults. In early infancy all children are inclined to be bow-legged. In their first efforts at walking they invariably press most of their weight on the outer edge of the foot and curve the toes inward, as if to grasp the surface on which the foot is placed. The instinct of prehension cannot be mistaken. It differs in degree in different races, and is vastly more pronounced in negro infants than in white ones.
There is another peculiar feature in the walk of the chimpanzee. The arms and legs do not alternate in motion with the same degree of regularity that they do in man or quadrupeds. This ape uses his arms more like crutches. They are moved forward, not quite, but almost at the same instant, and the motion of the legs is not at equal intervals. To be more explicit: the hands are placed almost opposite each other; the right foot is advanced about three times its length; the left foot is then placed about one length in front of the right; the arms are again moved; the right foot is again advanced about three lengths forward of the left; and the left again brought about one length in front of that. The same animal does not always use the same foot to make the long stride. It will be seen by this that each foot moves through the same space, and that, in a line, the tracks of either foot are the same distance apart; but the distance from the track of the right foot to that of the left is about three times as great as the distance from the track of the left foot to that of the right. Or the reverse may be the case. The distance from the track of either foot to the succeeding track of the other is never the same between the right and left tracks, except where the animal is walking at great leisure.
There is, perhaps, no animal more awkward than the chimpanzee, when he attempts to run. He sometimes swings his body with such force between his arms as to lose his balance and fall backward on the ground. Sometimes when he rights himself again, he is half his length backward of his starting point.