The Outline of Science Part 21

Some men, oftener than women, show on the inturned margin of the ear-trumpet or pinna, a little conical projection of great interest. It is a vestige of the tip of the pointed ear of lower mammals, and it is well named _Darwin's point_. It was he who described it as a "surviving symbol of the stirring times and dangerous days of man's animal youth."

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Physiological Proof of Man's Relationship with a Simian Stock

The everyday functions of the human body are practically the same as those of the anthropoid ape, and similar disorders are common to both.

Monkeys may be infected with certain microbes to which man is peculiarly liable, such as the bacillus of tuberculosis. Darwin showed that various human gestures and facial expressions have their counterparts in monkeys. The sneering curl of the upper lip, which tends to expose the canine tooth, is a case in point, though it may be seen in many other mammals besides monkeys--in dogs, for instance, which are at some considerable distance from the simian branch to which man's ancestors belonged.

When human blood is transfused into a dog or even a monkey, it behaves in a hostile way to the other blood, bringing about a destruction of the red blood corpuscles. But when it is transfused into a chimpanzee there is an harmonious mingling of the two. This is a very literal demonstration of man's blood-relationship with the higher apes. But there is a finer form of the same experiment. When the blood-fluid (or serum) of a rabbit, which has had human blood injected into it, is mingled with human blood, it forms a cloudy precipitate. It forms almost as marked a precipitate when it is mingled with the blood of an anthropoid ape. But when it is mingled with the blood of an American monkey there is only a slight clouding after a considerable time and no actual precipitate. When it is added to the blood of one of the distantly related "half-monkeys" or lemurs there is no reaction or only a very weak one. With the blood of mammals off the simian line altogether there is no reaction at all. Thus, as a distinguished anthropologist, Professor Schwalbe, has said: "We have in this not only a proof of the literal blood-relationship between man and apes, but the degree of relationship with the different main groups of apes can be determined beyond possibility of mistake." We can imagine how this modern line of experiment would have delighted Darwin.

[Ill.u.s.tration: THE GORILLA, INHABITING THE FOREST TRACT OF THE GABOON IN AFRICA

A full-grown individual stands about 5 feet high. The gait is shuffling, the strength enormous, the diet mainly vegetarian, the temper rather ferocious.]

Embryological Proof of Man's Relationship with a Simian Stock

In his individual development, man does in some measure climb up his own genealogical tree. Stages in the development of the body during its nine months of ante-natal life are closely similar to stages in the development of the anthropoid embryo. Babies born in times of famine or siege are sometimes, as it were, imperfectly finished, and sometimes have what may be described as monkeyish features and ways. A visit to an inst.i.tution for the care of children who show arrested, defective, or disturbed development leaves one sadly impressed with the risk of slipping down the rungs of the steep ladder of evolution; and even in adults the occurrence of serious nervous disturbance, such as "sh.e.l.l-shock," is sometimes marked by relapses to animal ways. It is a familiar fact that a normal baby reveals the past in its surprising power of grip, and the careful experiments of Dr. Louis Robinson showed that an infant three weeks old could support its own weight for over two minutes, holding on to a horizontal bar. "In many cases no sign of distress is evinced and no cry uttered, until the grasp begins to give way." This persistent grasp probably points back to the time when the baby had to cling to its arboreal mother. The human tail is represented in the adult by a fusion of four or five vertebrae forming the "coccyx"

at the end of the backbone, and is normally concealed beneath the flesh, but in the embryo the tail projects freely and is movable. Up to the sixth month of the ante-natal sleep the body is covered, all but the palms and soles, with longish hair (the lanugo), which usually disappears before birth. This is a stage in the normal development, which is reasonably interpreted as a recapitulation of a stage in the racial evolution. We draw this inference when we find that the unborn offspring of an almost hairless whale has an abundant representation of hairs; we must draw a similar inference in the case of man.

It must be noticed that there are two serious errors in the careless statement often made that man in his development is at one time like a little fish, at a later stage like a little reptile, at a later stage like a little primitive mammal, and eventually like a little monkey. The first error here is that the comparison should be made with _embryo_-fish, _embryo_-reptile, _embryo_-mammal, and so on. It is in the making of the embryos that the great resemblance lies. When the human embryo shows the laying down of the essential vertebrate characters, such as brain and spinal cord, then it is closely comparable to the embryo of a lower vertebrate at a similar stage. When, at a subsequent stage, its heart, for instance, is about to become a four-chambered mammalian heart, it is closely comparable to the heart of, let us say, a turtle, which never becomes more than three-chambered.

The point is that in the making of the organs of the body, say brain and kidneys, the embryo of man pursues a path closely corresponding to the path followed by the embryos of other backboned animals lower in the scale, but at successive stages it parts company with these, with the lowest first and so on in succession. A human embryo is never like a little reptile, but the developing organs pa.s.s through stages which very closely resemble the corresponding stages in lower types which are in a general way ancestral.

The second error is that every kind of animal, man included, has from the first a certain individuality, with peculiar characteristics which are all its own. This is expressed by the somewhat difficult word _specificity_, which just means that every species is itself and no other. So in the development of the human embryo, while there are close resemblances to the embryos of apes, monkeys, other mammals, and even, at earlier stages still, to the embryos of reptile and fish, it has to be admitted that we are dealing from first to last with a human embryo with peculiarities of its own.

[Ill.u.s.tration: "DARWIN'S POINT" ON HUMAN EAR (MARKED D.P.)

It corresponds to the tip (T) of the ear of an ordinary mammal, as shown in the hare's ear below. In the young orang the part corresponding to Darwin's point is still at the tip of the ear.]

[Ill.u.s.tration: _Photo: J. Russell & Sons._

PROFESSOR SIR ARTHUR KEITH, M.D., LL.D., F.R.S.

Conservator of the Museum and Hunterian Professor, Royal College of Surgeons of England. One of the foremost living anthropologists and a leading authority on the antiquity of man.]

[Ill.u.s.tration: _After T. H. Huxley (by permission of Messrs.

Macmillan)._

SKELETONS OF THE GIBBON, ORANG, CHIMPANZEE, GORILLA, MAN

Photographically reduced from diagrams of the natural size (except that of the gibbon, which was twice as large as nature) drawn by Mr.

Waterhouse Hawkins from specimens in the Museum of the Royal College of Surgeons.]

Every human being begins his or her life as a single cell--a fertilised egg-cell, a treasure-house of all the ages. For in this living microcosm, only a small fraction (1/125) of an inch in diameter, there is condensed--who can imagine how?--all the natural inheritance of man, all the legacy of his parentage, of his ancestry, of his long pre-human pedigree. Darwin called the pinhead brain of the ant the most marvellous atom of matter in the world, but the human ovum is more marvellous still. It has more possibilities in it than any other thing, yet without fertilisation it will die. The fertilised ovum divides and redivides; there results a ball of cells and a sack of cells; gradually division of labour becomes the rule; there is a laying down of nervous system and food-ca.n.a.l, muscular system and skeleton, and so proceeds what is learnedly called differentiation. Out of the apparently simple there emerges the obviously complex. As Aristotle observed more than two thousand years ago, in the developing egg of the hen there soon appears the beating heart! There is nothing like this in the non-living world.

But to return to the developing human embryo, there is formed from and above the embryonic food-ca.n.a.l a skeletal rod, which is called the notochord. It thrills the imagination to learn that this is the only supporting axis that the lower orders of the backboned race possess. The curious thing is that it does not become the backbone, which is certainly one of the essential features of the vertebrate race. The notochord is the supporting axis of the pioneer backboned animals, namely the Lancelets and the Round-mouths (Cyclostomes), such as the Lamprey. They have no backbone in the strict sense, but they have this notochord. It can easily be dissected out in the lamprey--a long gristly rod. It is surrounded by a sheath which becomes the backbone of most fishes and of all higher animals. The interesting point is that although the notochord is only a vestige in the adults of these types, it is never absent from the embryo. It occurs even in man, a short-lived relic of the primeval supporting axis of the body. It comes and then it goes, leaving only minute traces in the adult. We cannot say that it is of any use, unless it serves as a stimulus to the development of its subst.i.tute, the backbone. It is only a piece of preliminary scaffolding, but there is no more eloquent instance of the living hand of the past.

One other instance must suffice of what Professor Lull calls the wonderful changes wrought in the dark of the ante-natal period, which recapitulate in rapid abbreviation the great evolutionary steps which were taken by man's ancestors "during the long night of the geological past." On the sides of the neck of the human embryo there are four pairs of slits, the "visceral clefts," openings from the beginning of the food-ca.n.a.ls to the surface. There is no doubt as to their significance.

They correspond to the gill-slits of fishes and tadpoles. Yet in reptiles, birds, and mammals they have no connection with breathing, which is their function in fishes and amphibians. Indeed, they are not of any use at all, except that the first becomes the Eustachian tube bringing the ear-pa.s.sage into connection with the back of the mouth, and that the second and third have to do with the development of a curious organ called the thymus gland. Persistent, nevertheless, these gill-slits are, recalling even in man an aquatic ancestry of many millions of years ago.

When all these lines of evidence are considered, they are seen to converge in the conclusion that man is derived from a simian stock of mammals. He is solidary with the rest of creation. To quote the closing words of Darwin's _Descent of Man_:

We must, however, acknowledge, as it seems to me, that man with all his n.o.ble qualities, with sympathy which feels for the most debased, with benevolence which extends not only to other men but to the humblest living creature, with his G.o.d-like intellect, which has penetrated into the movements and const.i.tution of the solar system--with all these exalted powers--man still bears in his bodily frame the indelible stamp of his lowly origin.

We should be clear that this view does not say more than that man sprang from a stock common to him and to the higher apes. Those who are repelled by the idea of man's derivation from a simian type should remember that the theory implies rather more than this, namely, that man is the outcome of a genealogy which has implied many millions of years of experimenting and sifting--the groaning and travailing of a whole creation. Speaking of man's mental qualities, Sir Ray Lankester says: "They justify the view that man forms a new departure in the gradual unfolding of Nature's predestined plan." In any case, we have to try to square our views with the facts, not the facts with our views, and while one of the facts is that man stands unique and apart, the other is that man is a scion of a progressive simian stock. Naturalists have exposed the pit whence man has been digged and the rock whence he has been hewn, but it is surely a heartening encouragement to know that it is an ascent, not a descent, that we have behind us. There is wisdom in Pascal's maxim:

It is dangerous to show man too plainly how like he is to the animals, without, at the same time, reminding him of his greatness.

It is equally unwise to impress him with his greatness and not with his lowliness. It is worse to leave him in ignorance of both. But it is very profitable to recognise the two facts.

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Man's Pedigree

The facts of anatomy, physiology, and embryology, of which we have given ill.u.s.trations, all point to man's affiliation with the order of monkeys and apes. To this order is given the name Primates, and our first and second question must be when and whence the Primates began. The rock record answers the first question: the Primates emerged about the dawn of the Eocene era, when gra.s.s was beginning to cover the earth with a garment. Their ancestral home was in the north in both hemispheres, and then they migrated to Africa, India, Malay, and South America. In North America the Primates soon became extinct, and the same thing happened later on in Europe. In this case, however, there was a repeopling from the South (in the Lower Miocene) and then a second extinction (in the Upper Pliocene) before man appeared. There is considerable evidence in support of Professor R. S. Lull's conclusion, that in Southern Asia, Africa, and South America the evolution of Primates was continuous since the first great southward migration, and there is, of course, an abundant modern representation of Primates in these regions to-day.

As to the second question: Whence the Primates sprang, the answer must be more conjectural. But it is a reasonable view that Carnivores and Primates sprang from a common Insectivore stock, the one order diverging towards flesh-eating and hunting on the ground, the other order diverging towards fruit-eating and arboreal habits. There is no doubt that the Insectivores (including shrews, tree-shrews, hedgehog, mole, and the like) were very plastic and progressive mammals.

What followed in the course of ages was the divergence of branch after branch from the main Primate stem. First there diverged the South American monkeys on a line of their own, and then the Old World monkeys, such as the macaques and baboons. Ages pa.s.sed and the main stems gave off (in the Oligocene period) the branch now represented by the small anthropoid apes--the gibbon and the siamang. Distinctly later there diverged the branch of the large anthropoid apes--the gorilla, the chimpanzee, and the orang. That left a generalised humanoid stock separated off from all monkeys and apes, and including the immediate precursors of man. When this sifting out of a generalised humanoid stock took place remains very uncertain, some authorities referring it to the Miocene, others to the early Pliocene. Some would estimate its date at half a million years ago, others at two millions! The fact is that questions of chronology do not as yet admit of scientific statement.

[Ill.u.s.tration: SIDE-VIEW OF SKULL OF MAN (M) AND GORILLA (G)

Notice in the gorilla's skull the protrusive face region, the big eyebrow ridges, the much less domed cranial cavity, the ma.s.sive lower jaw, the big canine teeth. Notice in man's skull the well-developed forehead, the domed and s.p.a.cious cranial cavity, the absence of any snout, the chin process, and many other marked differences separating the human skull from the ape's.]

[Ill.u.s.tration: THE SKULL AND BRAIN-CASE OF PITHECANTHROPUS, THE JAVA APE-MAN, AS RESTORED. BY J. H. McGREGOR FROM THE SCANTY REMAINS

The restoration shows the low, retreating forehead and the prominent eyebrow ridges.]

[Ill.u.s.tration: SUGGESTED GENEALOGICAL TREE OF MAN AND ANTHROPOID APES

From Sir Arthur Keith; the lettering to the right has been slightly simplified.]

We are on firmer, though still uncertain, ground when we state the probability that it was in Asia that the precursors of man were separated off from monkeys and apes, and began to be terrestrial rather than arboreal. Professor Lull points out that Asia is nearest to the oldest known human remains (in Java), and that Asia was the seat of the most ancient civilisations and the original home of many domesticated animals and cultivated plants. The probability is that the cradle of the human race was in Asia.

Man's Arboreal Apprenticeship

At this point it will be useful to consider man's arboreal apprenticeship and how he became a terrestrial journeyman. Professor Wood Jones has worked out very convincingly the thesis that man had no direct four-footed ancestry, but that the Primate stock to which he belongs was from its first divergence arboreal. He maintains that the leading peculiarities of the immediate precursors of man were wrought out during a long arboreal apprenticeship. The first great gain of arboreal life on bipedal erect lines (not after the quadrupedal fashion of tree-sloths, for instance) was the emanc.i.p.ation of the hand. The foot became the supporting and branch-gripping member, and the hand was set free to reach upward, to hang on by, to seize the fruit, to lift it and hold it to the mouth, and to hug the young one close to the breast.

The hand thus set free has remained plastic--a generalised, not a specialised member. Much has followed from man's "handiness."

The arboreal life had many other consequences. It led to an increased freedom of movement of the thigh on the hip joint, to muscular arrangements for balancing the body on the leg, to making the backbone a supple yet stable curved pillar, to a strongly developed collar-bone which is only found well-formed when the fore-limb is used for more than support, and to a power of "opposing" the thumb and the big toe to the other digits of the hand and foot--an obvious advantage for branch-gripping. But the evolution of a free hand made it possible to dispense with protrusive lips and gripping teeth. Thus began the recession of the snout region, the a.s.sociated enlargement of the brain-box, and the bringing of the eyes to the front. The overcrowding of the teeth that followed the shortening of the snout was one of the taxes on progress of which modern man is often reminded in his dental troubles.

Another acquisition a.s.sociated with arboreal life was a greatly increased power of turning the head from side to side--a mobility very important in locating sounds and in exploring with the eyes.

Furthermore, there came about a flattening of the chest and of the back, and the movements of the midriff (or diaphragm) came to count for more in respiration than the movements of the ribs. The sense of touch came to be of more importance and the sense of smell of less; the part of the brain receiving tidings from hand and eye and ear came to predominate over the part for receiving olfactory messages. Finally, the need for carrying the infant about among the branches must surely have implied an intensification of family relations, and favoured the evolution of gentleness.

[Ill.u.s.tration: _Photo: New York Zoological Park._

THE GIBBON IS LOWER THAN THE OTHER APES AS REGARDS ITS SKULL AND DENt.i.tION, BUT IT IS HIGHLY SPECIALIZED IN THE ADAPTATION OF ITS LIMBS TO ARBOREAL LIFE]