It is very possible that the reader may still entertain a doubt as to the probability of man having come by his upright posture and plantigrade gait in the manner explained in the last chapter. In this one I propose to discuss with him some very peculiar features of man's body which are concerned with posture, and which afford additional evidence that the human stock was not always adapted for the erect posture. The first of these is the human tail. It is not a matter one cares to lay emphasis on, yet for the sake of truth it must be admitted that man is the descendant of a tailed primate. The tail is a direct prolongation of the backbone; all those segments or vertebrae which lie beyond the sacral verbe-brae—the ones to which the hind limbs are attached—are tail or caudal vertebras. In man, these vertebrae, four or five in number and vestigial in size and form, are buried beneath the skin. In the human embryo up to the sixth week, the tail projects on the surface of the body; its projection is best seen when the embryo is in the third and fourth weeks of development. Even at birth a depression in the skin marks the point at which the tail sinks within the body. It is not uncommon to find during dissection of the human body vestigial muscles passing to the coccyx, which represent the tail muscles of lower animals. Well authenticated cases are on record of children who have been born with true tails. Such cases are rare, and the tails are little better than soft string-like appendages, but their structure, and the fact that they form a continuation of the backbone, leave no doubt as to their true nature.
Many years ago critics of Darwin often twitted his supporters on the subject of man's tail; they regarded the theory of a human tail as a joke. A tailless condition is not confined to man ; in the anthropoids, both great and small, the tail has disappeared to even a greater degree than in man. In some monkeys the tail has been reduced to a short projecting stump, but only in man and the anthropoids can the tail be said to be completely hidden and reduced to the condition of a coccyx. When it is remembered that it is only these higher primates which have attained the erect or upright posture it will be at once suspected that the disappearance of the tail is a result of a change of posture. There can be no doubt that this is the case. We have seen that when a monkey is held upright its viscera gravitate downwards and need support from below. The muscles which close the hinder end of the body are the muscles which depress the tail; by depressing the tail the monkey can support or shut in the contents of the abdomen. In man, the great anthropoids, and in the gibbon we find the muscles which depress the tail spread out as a muscular hammock across the pelvis to support the viscera. The exact function of the tail in monkeys we do not know accurately; in South American forms it is used as an extra hand ; in Old World monkeys it seems to serve as a balancing-rod, for one notices those with long tails now holding them aloft, at other times trailing them behind, first on one side of the branch they are walking on, then on the other. With the evolution of the upright posture the tail became useless as a balancing organ; the centre of the gravity of the body became then quite altered. The muscles which depressed the tail were needed for the support of the abdominal organs, and hence the tail became useless in the new economy which was established and became buried or coccygeal in form. The actual process which leads to the disappearance of useless organs we do not know fully, but we do know that they vanish, usually as in the present case, leaving some mark or trace behind. Thus the disappearance of the tail did not take place when man as we know him now was being evolved ; it was even suppressed before the great anthropoids came on the scene. Amongst all the animals now living, the gibbon is the most primitive tailless form, and it was probably during its evolution from a monkeylike form that the tail was lost. Seeing how long the tail has ceased to be a functional organ in the ancestry of man, it is a matter of astonishment, not that it is rarely developed, but that it should reappear at all.
Occasionally men are born with the organs within the abdomen fixed and arranged exactly as they are in horizontally placed— or, to use a better term—pronograde monkeys. Surgeons are well aware of the occurrence of such anomalies, for in these men the bowel being loosely attached by its mesentery, is apt to become twisted on itself, thus causing obstruction. In the earlier stages of development of the human embryo the bowel is attached as in a pronograde monkey, but in the later months the adaptations seen in upright or orthograde primates take place. The small bowel in pronograde animals is attached by a mesentery or sheet of membrane, shaped somewhat like a fan ; when the human foetus enters the orthograde or upright stage of development one side of the mesentery becomes applied to the posterior wall of the abdomen, and thus the bowel is more closely bound down. It would be impossible to explain the facts relating to the fixation of the abdominal viscera unless we suppose that at one stage of evolution, the ancestry of man was pronograde in its posture.
Very little has been said as yet concerning the effect of the upright or orthograde posture on the organs within the chest—the heart and lungs. It is quite evident, however, that when a pronograde animal is held erect not only do its abdominal viscera tend to sink down, but so also do those within the thorax. In the pronograde ape there is a space between the heart and the diaphragm which is filled or occupied by a process or lobe (azygos lobe) of the base of the right lung. In orthograde animals, such as man and the anthropoids, the heart comes to rest on the upper surface of the diaphragm and the azygos lobe disappears. A rudiment of it can always be seen, and occasionally it is of some size, and projects inwards between the heart and diaphragm occupying the same position as in pronograde apes. We cannot explain the presence of a vestige of the azygos lobe unless we suppose that man had passed through a pronograde stage.