Part 2 (2/2)
It is of interest to find that all the existing large apes are arboreal, the gorilla being the least so, probably on account of its weight. Though they all descend at times to the ground, their awkward motion on the surface shows them to be out of their element, while they move with ease and rapidity in the trees. The organization of man renders it questionable if his primeval ancestor was arboreal to any similar extent. The indications would seem to be that it made the ground its habitual place of residence at an early period in its history, and that the result of this new habit and of its erect att.i.tude was a change in the relative length of its limbs.
That this animal dwelt mainly in trees in the first stage of its existence, and possessed a powerful grasping power in its hands, we have corroborative evidence in recent studies of child life. The human infant, in its earliest days of life, displays a remarkable grasping power, being able to sustain its weight with its hands for a number of seconds, or a minute or more, at an age when its other muscles are flabby and powerless. It appears in this to repeat a habit normal to the ancestral infant, an instinct developed to prevent a fall from its home among the boughs.
Yet it is doubtful if the man-ape long remained a specially arboreal animal. The varied length of arm in the anthropoid apes was doubtless of early origin, and in all probability man's ancestor had originally a shorter arm than its related species. If so, this must have rendered it less agile in trees than other forms. If we could see this ancient creature in its arboreal home, we should probably find it more inclined to stand erect than the other apes, walking on a lower limb, and steadying itself by grasping an upper limb. This would be a more natural and easy mode of progression to a short-armed animal than the crouching att.i.tude of the orang or the swinging motion of the gibbon, and its effect would be to make the erect att.i.tude to a large extent habitual with this animal.
In short, man's ancestor may have become in considerable measure a biped while still largely a dweller in the trees, and to that degree set its arms free for other duties than that of locomotion. Like the other apes, it probably often descended to the ground, where its habit of walking erect on the boughs rendered the biped walk an easy one, or where this habit may have been originally acquired. While this is conjectural, it is supported by facts of organization and existing habit, and for the reasons given it seems highly probable that the ancestor of man took to a land residence at an early period in its history, climbing again for food or safety, but dwelling more and more habitually on the earth's surface. Even at this remote era it may have become essentially human in organization, its subsequent changes being mainly in brain development, and only to a minor extent in physical form and structure.
Fossil apes have not been found farther back than the Miocene Age of geology. It is quite probable, however, that they may yet be found in Eocene strata, since examples of their highest representatives, the anthropoid or manlike apes, have been found in Miocene rocks. The fact that these large apes are now few in number of species, is no proof that many forms of them may not have formerly existed, and among these we may cla.s.s the ancestor of man.
V
THE FREEDOM OF THE ARMS
Man's ancestor is by no means the only form of ape that has made the earth's surface its place of residence. The baboon is one example of a number of forms that dwell habitually upon the ground, though they have not lost their agility in climbing. But these species have returned to the quadruped habit, to which the equal length of their limbs adapts them. All the anthropoid apes dwell to some extent upon the ground, but these can neither be called quadrupeds nor bipeds, their usual mode of progression being an awkward compromise between the two. The same may be said of one of the lemurs, the propithecus, the only member of its tribe that attempts to move in the erect att.i.tude. It does not walk, however, but progresses by a series of jumps, its arms being held erect, as if for balancing.
Of the apes, though many can stand upright, the gibbon is the only one that attempts to walk in this position. This is a true walk, though not a very graceful one. The animal maintains a fairly upright posture, but walks with a waddling motion, its body rocking from side to side. Its soles are placed flat on the ground, with the great toes spread outward. Its arms either hang loosely by its side, are crossed over its head, or are held aloft, swaying like balancing poles and ready to seize any overhead support. Its walk is quickly changed to a different motion if any occasion for haste arises. At once its long arms are dropped to the ground, the knuckles closed, and it progresses by a swinging or leaping motion, the body remaining nearly erect, but being swung between the arms.
None of the other anthropoid apes ever walk erect, though they a.s.sume at times the upright posture. But though they use all their limbs as walking organs, they show no tendency to revert to the habit of the quadrupeds. Their motion is like that of the gibbon when in haste, a series of jumps or swings between the supporting arms. The shortness of their arms, however, prevents them from standing erect, like the gibbon, in doing this; and they bend forward to a degree depending on the length of their arms, the chimpanzee the most, the orang the least.
As a rule, the flat sole of the foot is set on the ground, with the toes extended, as in man, but the toes are sometimes doubled under in walking. The orang rarely touches the ground with the sole or the closed toes, but walks on the outer edge of the foot, the feet being bent inward as if clasping the rounded sides of a bough. The other species have a tendency in the same direction, the legs being bowed and the gait rolling. In using the hands in walking, the closed knuckles are usually placed on the ground, though occasionally the open palm is employed. The whole movement of these animals is strikingly awkward, and goes to indicate that there can be no satisfactory compromise between life in the tree and on the ground.
The significant fact in these attempts to walk is that none of the anthropoid apes show any inclination to revert to the quadruped habit.
Their att.i.tude is in all cases an approach toward the erect one, which posture is attained by the gibbon. The arms are used not as walking but as swinging organs. Evidently their mode of life in the trees has overcome all tendency toward the quadruped motion in these apes and developed a tendency toward the biped. But none of them have gained the muscular development of the leg known as the calf, nor an adjustment of the joints to the erect att.i.tude, since none but the gibbon walks erect, and it does so only at occasional intervals.
The conclusion to be derived from all this is that the man-ape was in its early days much more truly a biped than are any of the species named. Like them, it had no tendency to revert to the quadruped habit.
The shortness of its arms was unsuited to this, while rendering it impossible for the animal to progress in the semi-erect, swinging fas.h.i.+on of the other anthropoid apes. As a result of its bodily formation, it may have begun to walk erect at a very remote date, with a consequent straightening of the joints and muscular development of the legs. When this condition was fully attained, it was practically a man in physical conformation, though mentally still an ape, and with a long development of the brain to pa.s.s through before it could reach the human level of mind.
The far-reaching conclusions here reached are all based on one important fact, the shortness of man's arms as compared with the disproportionate length of arm in the anthropoid apes. This, for the reasons given, rendered the adaptation of the man-ape to life in the trees inferior to that of the long-armed apes; while, as has just been said, it unfitted it to walk on the ground either as a quadruped or in the jumping method of its fellow anthropoids. In short, the biped att.i.tude was much the best suited to its organization and the one it was most likely to a.s.sume. This once adopted as its habitual posture, efficiency in walking would be gained by practice.
When once this animal became a ground walker, its facility of motion in the trees was in a measure lost. When the feet became accustomed to the flat surface of the ground, they became less capable of grasping the rounded surface of the bough. Fitness to the one situation entailed loss of fitness to the other. The feet of the apes can clasp the bough firmly, by curving around its opposite sloping sides, and to this these animals doubtless owe their bowed legs and their disposition to walk on the outer edge of the foot. This disposition the man-ape lost as its foot fitted itself to the surface of the ground. It was probably retained in a measure by the young, after it had been lost by the mature form, and is still manifested in the position of the foot in the human embryo.
These considerations bring us to an important question: Why did the man-ape gain a length of arm not the best suited to its arboreal habitat? Why, in fact, do changes in physical structure ever take place?
How does an animal succeed in pa.s.sing from one mode of life to another, when during the transition period it is imperfectly adapted to either, and therefore at a seeming disadvantage in the struggle for existence?
The study of animal development has given rise to certain difficult problems of this character, some of which have been solved by showing that the supposed disadvantage did not arise, or that it was balanced by some equal advantage. In this way a considerable gap in life conditions has perhaps occasionally been crossed. Small gaps have doubtless been frequently pa.s.sed over in the same manner.
In the case of the anthropoid apes, we perceive a considerable variation in the length of the arms, from the very long arms of the gibbon to the comparatively short ones of the chimpanzee. These differences are probably the result of some difference in their life habits, and accord with the possibility of a still shorter arm in the man-ape. There is, however, some reason to believe, as we shall show later on, that the arm of this animal was longer and the leg shorter than in man himself, their comparative length perhaps not differing greatly from that of the chimpanzee. Aside from all other considerations, the use of the legs as the sole organs of locomotion could not well fail to produce this result, the legs growing longer and stronger in consequence of the increased duty laid upon them, and the arms growing shorter and weaker through their release from duty in locomotion. The case does not differ in character from those of the dinosauria and the kangaroos, in both of which instances a release of the arms from duty in walking was followed by a considerable decrease in length and strength, while the legs grew proportionally stronger.
If any disadvantage attended the shortening of the arms of the man-ape, to the extent that this may have taken place in the tree, it was probably correlated with some advantage. In the various instances of short-armed animals cited this appears to have been the case, and it was probably so in man's ancestral form. While the hands continued useful in grasping and enabling the animal to maintain its place on the boughs, they may have been gradually diverted to some other service, with the result that the animal found the tree less desirable than before as a place of residence and sought the ground instead. This would be particularly the case if the new duty was one best exercised upon the ground.
Shall we offer a suggestion as to this new use? Such changes are usually the result of some change of habit in the animal, frequently one that has to do with its food. Change of diet or of the mode of obtaining food is the most potent influencing cause of change of habit in animals, and the one that first calls for consideration.
The apes are frugivorous animals, though not exclusively so. Carnivorous tendencies are displayed by many of them. They rob birds' nests of their eggs and young, they capture and devour snakes and other small animals.
In zoological gardens monkeys are often observed to catch and eat mice.
It is evident that many of them might readily become carnivorous to a large extent under suitable conditions. The large apes are usually frugivorous, but some of them eat animal food. This is the case with both the chimpanzee and the gorilla. The latter, while living usually on fruit and often making havoc in the sugar-cane plantations and rice-fields of the natives, also eats birds and their eggs, small mammals and reptiles, and is said to devour large animals when found dead, though it does not attempt to kill them for food. The young gorilla which was kept in captivity at Berlin became quite omnivorous in its diet.
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