Rapid Colour-change.
Bony flat-fishes, like plaice and sole, have a remarkable power of adjusting their hue and pattern to the surrounding gravel and sand, so that it is difficult to find them even when we know that they are there. It must be admitted that they are also very quick to get a sprinkling of sand over their upturned side, so that only the eyes are left showing. But there is no doubt as to the exactness with which they often adjust themselves to be like a little piece of the substratum on which they lie; they will do this within limits in experimental conditions when they are placed on a quite artificial floor. As these fishes are very palatable and are much sought after by such enemies as cormorants and otters, it is highly probably that their power of self-effacement often saves their life. And it may be effected within a few minutes, in some cases within a minute.
In these self-effacing flat-fishes we know with some precision what happens. The adjustment of colour and pattern is due to changes in the size, shape, and position of mobile pigment-cells (chromatoph.o.r.es) and the skin. But what makes the pigment-cells change? The fact that a blind flat-fish does not change its colour gives us the first part of the answer. The colour and the pattern of the surroundings must affect the eye. The message travels by the optic nerve to the brain; from the brain, instead of pa.s.sing down the spinal cord, the message travels down the chain of sympathetic ganglia. From these it pa.s.ses along the nerves which comes out of the spinal cord and control the skin. Thus the message reaches the colour-cells in the skin, and before you have carefully read these lines the flat-fish has slipped on its Gyges ring and become invisible.
The same power of rapid colour-change is seen in cuttlefishes, where it is often an expression of nervous excitement, though it sometimes helps to conceal. It occurs with much subtlety in the aesop prawn, Hippolyte, which may be brown on a brown seaweed, green on sea-lettuce or sea-gra.s.s, red on red seaweed, and so on through an extensive repertory.
According to the nature of the background, [Professor Gamble writes] so is the mixture of the pigments compounded so as to form a close reproduction both of its colour and its pattern. A sweep of the shrimp net detaches a battalion of these sleeping prawns, and if we turn the motley into a dish and give a choice of seaweed, each variety after its kind will select the one with which it agrees in colour, and vanish. Both when young and when full-grown, the aesop prawn takes on the colour of its immediate surroundings. At nightfall Hippolyte, of whatever colour, changes to a transparent azure blue: its stolidity gives place to a nervous restlessness; at the least tremor it leaps violently, and often swims actively from one food-plant to another. This blue fit lasts till daybreak, and is then succeeded by the prawn"s diurnal tint.
Thus, Professor Gamble continues, the colour of an animal may express a nervous rhythm.
[Ill.u.s.tration: Photo: J. J. Ward, F.E.S.
PROTECTIVE RESEMBLANCE.
Hawk Moth, settled down on a branch, and very difficult to detect as long as it remains stationary. Note its remarkable sucking tongue, which is about twice the length of its body. The tongue can be quickly coiled up and put safely away beneath the lower part of the head.]
[Ill.u.s.tration: WHEN ONLY A FEW DAYS OLD, YOUNG BITTERN BEGIN TO STRIKE THE SAME ATt.i.tUDE AS THEIR PARENTS THRUSTING THEIR BILLS UPWARDS AND DRAWING THEIR BODIES UP SO THAT THEY RESEMBLE A BUNCH OF REEDS The soft browns and blue-greens harmonise with the dull sheaths of the young reeds; the nestling bittern is thus completely camouflaged.]
The Case of Chameleons.
The highest level at which rapid colour-change occurs is among lizards, and the finest exhibition of it is among the chameleons. These quaint creatures are characteristic of Africa; but they occur also in Andalusia, Arabia, Ceylon, and Southern India. They are adapted for life on trees, where they hunt insects with great deliberateness and success. The protrusible tongue, ending in a sticky club, can be shot out for about seven inches in the common chameleon. Their hands and feet are split so that they grip the branches firmly, and the prehensile tail rivals a monkey"s. When they wish they can make themselves very slim, contracting the body from side to side, so that they are not very readily seen. In other circ.u.mstances, however, they do not practise self-effacement, but the very reverse. They inflate their bodies, having not only large lungs, but air-sacs in connection with them. The throat bulges; the body sways from side to side; and the creature expresses its sentiments in a hiss. The power of colour-change is very remarkable, and depends partly on the contraction and expansion of the colour-cells (chromatoph.o.r.es) in the under-skin (or dermis) and partly on close-packed refractive granules and crystals of a waste-product called guanin. The repertory of possible colours in the common chameleon is greater than in any other animal except the aesop prawn. There is a legend of a chameleon which was brown in a brown box, green in a green box, and blue in a blue box, and died when put into one lined with tartan; and there is no doubt that one and the same animal has a wide range of colours. The so-called "chameleon" (Anolis) of North America is so sensitive that a pa.s.sing cloud makes it change its emerald hue.
There is no doubt that a chameleon may make itself more inconspicuous by changing its colour, being affected by the play of light on its eyes. A bright-green hue is often seen on those that are sitting among strongly illumined green leaves. But the colour also changes with the time of day and with the animal"s moods. A sudden irritation may bring about a rapid change; in other cases the transformation comes about very gradually. When the colour-change expresses the chameleon"s feelings it might be compared to blushing, but that is due to an expansion of the arteries of the face, allowing more blood to get into the capillaries of the under-skin. The case of the chameleon is peculiarly interesting because the animal has two kinds of tactics--self-effacement on the one hand and bluffing on the other. There can be little doubt that the power of colour-change sometimes justifies itself by driving off intruders. Dr. Cyril Crossland observed that a chameleon attacked by a fox-terrier "turned round and opened its great pink mouth in the face of the advancing dog, at the same time rapidly changing colour, becoming almost black. This ruse succeeded every time, the dog turning off at once." In natural leafy surroundings the startling effect would be much greater--a sudden throwing off of the mantle of invisibility and the exposure of a conspicuous black body with a large red mouth.
-- 4.
Likeness to Other Things.
Dr. H. O. Forbes tells of a flat spider which presents a striking resemblance to a bird"s dropping on a leaf. Years after he first found it he was watching in a forest in the Far East when his eye fell on a leaf before him which had been blotched by a bird. He wondered idly why he had not seen for so long another specimen of the bird-dropping spider (Ornithoscatoides decipiens), and drew the leaf towards him. Instantaneously he got a characteristic sharp nip; it was the spider after all! Here the colour-resemblance was enhanced by a form-resemblance.
[Ill.u.s.tration: A. PROTECTIVE COLORATION OR CAMOUFLAGING, GIVING ANIMALS A GARMENT OF INVISIBILITY.
At the foot of the plate is a Nightjar, with plumage like bark and withering leaves; to the right, resting on a branch, is shown a Chameleon in a green phase amid green surroundings; the insects on the reeds are Locusts; while a green Frog, merged into its surroundings, rests on a leaf near the centre at the top of the picture.
B. ANOTHER EXAMPLE OF PROTECTIVE COLORATION OR CAMOUFLAGE.
A sh.o.r.e scene showing Trout in the pool almost invisible against their background. The Stone Curlews, both adult and young, are very inconspicuous among the stones on the beach.]
But why should it profit a spider to be like a bird-dropping? Perhaps because it thereby escapes attention; but there is another possibility. It seems that some b.u.t.terflies, allied to our Blues, are often attracted to excrement.i.tious material, and the spider Dr. Forbes observed had actually caught its victim. This is borne out by a recent observation by Dr. D. G. H. Carpenter, who found a Uganda bug closely resembling a bird-dropping on sand. The bug actually settled down on a bird-dropping on sand, and caught a blue b.u.t.terfly which came to feed there!
Some of the walking-stick insects, belonging to the order of crickets and gra.s.shoppers (Orthoptera), have their body elongated and narrow, like a thin dry branch, and they have a way of sticking out their limbs at abrupt and diverse angles, which makes the resemblance to twigs very close indeed. Some of these quaint insects rest through the day and have the remarkable habit of putting themselves into a sort of kataleptic state. Many creatures turn stiff when they get a shock, or pa.s.s suddenly into new surroundings, like some of the sand-hoppers when we lay them on the palm of our hand; but these twig-insects put themselves into this strange state. The body is rocked from side to side for a short time, and then it stiffens. An advantage may be that even if they were surprised by a bird or a lizard, they will not be able to betray themselves by even a tremor. Disguise is perfected by a remarkable habit, a habit which leads us to think of a whole series of different ways of lying low and saying nothing which are often of life-preserving value. The top end of the series is seen when a fox plays "possum.
The leaf-b.u.t.terfly Kallima, conspicuously coloured on its upper surface, is like a withered leaf when it settles down and shows the under side of its wings. Here, again, there is precise form-resemblance, for the nervures on the wings are like the mid-rib and side veins on a leaf, and the touch of perfection is given in the presence of whitish spots which look exactly like the discolorations produced by lichens on leaves. An old entomologist, Mr. Jenner Weir, confessed that he repeatedly pruned off a caterpillar on a bush in mistake for a superfluous twig, for many brownish caterpillars fasten themselves by their posterior claspers and by an invisible thread of silk from their mouth, and project from the branch at a twig-like angle. An insect may be the very image of a sharp p.r.i.c.kle or a piece of soft moss; a spider may look precisely like a tiny k.n.o.b on a branch or a fragment of lichen; one of the sea-horses (Phyllopteryx) has frond-like ta.s.sels on various parts of its body, so that it looks extraordinarily like the seaweeds among which it lives. In a few cases, e.g. among spiders, it has been shown that animals with a special protective resemblance to something else seek out a position where this resemblance tells, and there is urgent need for observations bearing on this selection of environment.
-- 5.
Mimicry in the True Sense.
It sometimes happens that in one and the same place there are two groups of animals not very nearly related which are "doubles" of one another. Investigation shows that the members of the one group, always in the majority, are in some way specially protected, e.g. by being unpalatable. They are the "mimicked." The members of the other group, always in the minority, have not got the special protection possessed by the others. They are the "mimickers," though the resemblance is not, of course, a.s.sociated with any conscious imitation. The theory is that the mimickers live on the reputation of the mimicked. If the mimicked are left alone by birds because they have a reputation for unpalatability, or because they are able to sting, the mimickers survive--although they are palatable and stingless. They succeed, not through any virtue of their own, but because of their resemblance to the mimicked, for whom they are mistaken. There are many cases of mimetic resemblance so striking and so subtle that it seems impossible to doubt that the thing works; there are other cases which are rather far-fetched, and may be somewhat of the nature of coincidences. Thus although Mr. Bates tells us that he repeatedly shot humming-bird moths in mistake for humming-birds, we cannot think that this is a good ill.u.s.tration of mimicry. What is needed for many cases is what is forthcoming for some, namely, experimental evidence, e.g. that the unpalatable mimicked b.u.t.terflies are left in relative peace while similar palatable b.u.t.terflies are persecuted. It is also necessary to show that the mimickers do actually consort with the mimicked. Some beetles and moths are curiously wasplike, which may be a great advantage; the common drone-fly is superficially like a small bee; some harmless snakes are very like poisonous species; and Mr. Wallace maintained that the powerful "friar-birds" of the Far East are mimicked by the weak and timid orioles. When the model is unpalatable or repulsive or dangerous, and the mimic the reverse, the mimicry is called "Batesian" (after Mr. Bates), but there is another kind of mimicry called Mullerian (after Fritz Muller) where the mimic is also unpalatable. The theory in this case is that the mimicry serves as mutual a.s.surance, the members of the ring getting on better by consistently presenting the same appearance, which has come to mean to possible enemies a signal, Noli me tangere ("Leave me alone"). There is nothing out of the question in this theory, but it requires to be taken in a critical spirit. It leads us to think of "warning colours," which are the very opposite of the disguises which we are now studying. Some creatures like skunks, magpies, coral-snakes, cobras, brightly coloured tree-frogs are obtrusive rather than elusive, and the theory of Alfred Russel Wallace was that the flaunting conspicuousness serves as a useful advertis.e.m.e.nt, impressing itself on the memories of inexperienced enemies, who soon learn to leave creatures with "warning colours" alone. In any case it is plain that an animal which is as safe as a wasp or a coral-snake can afford to wear any suit of clothes it likes.
[Ill.u.s.tration: DEAD-LEAF b.u.t.tERFLY (Kallima Inachis) FROM INDIA.
It is conspicuous on its upper surface, but when it settles down on a twig and shows the underside of its wings it is practically invisible. The colouring of the under surface of the wings is like that of the withering leaf; there are spots like fungas spots; and the venation of the wings suggests the mid-rib and veins of the leaf. A, showing upper surface; B, showing under surface; C, a leaf.]
[Ill.u.s.tration: PROTECTIVE RESEMBLANCE BETWEEN A SMALL SPIDER (to the left) AND AN ANT (to the right) As ants are much dreaded, it is probably profitable to the spider to be like an ant. It will be noted that the spider has four pairs of legs and no feelers, whereas the ant has three pairs of legs and a pair of feelers.]
[Ill.u.s.tration: Photo: J. J. Ward, F.E.S.
THE WASP BEETLE, WHICH, WHEN MOVING AMONGST THE BRANCHES GIVES A WASP-LIKE IMPRESSION]
[Ill.u.s.tration: HERMIT-CRAB WITH PARTNER SEA-ANEMONES.
Hermit-crabs hide their soft tail in the sh.e.l.l of a whelk or some other sea-snail. But some hermit-crabs place sea-anemones on the back of their borrowed sh.e.l.l. The sea-anemones mask the hermit-crab and their tentacles can sting. As for the sea-anemones, they are carried about by the hermit-crab and they get crumbs from its table. This kind of mutually beneficial external partnership is called commensalism, i.e. eating at the same table.]
[Ill.u.s.tration: Photo: G. P. Duffus.
CUCKOO-SPIT.
The white ma.s.s in the centre of the picture is a soapy froth which the young frog-hopper makes, and within which it lies safe both from the heat of the sun and almost all enemies. After sojourning for a time in the cuckoo-spit, the frog-hopper becomes a winged insect.]
Masking.
The episode in Scottish history called "The Walking Wood of Birnam," when the advancing troop masked their approach by cutting down branches of the trees, has had its counterpart in many countries. But it is also enacted on the seash.o.r.e. There are many kinds of crabs that put on disguise with what looks like deliberateness. The sand-crab takes a piece of seaweed, nibbles at the end of it, and then rubs it on the back of the carapace or on the legs so that it fixes to the bristles. As the seaweed continues to live, the crab soon has a little garden on its back which masks the crab"s real nature. It is most effective camouflaging, but if the crab continues to grow it has to moult, and that means losing the disguise. It is then necessary to make a new one. The crab must have on the sh.o.r.e something corresponding to a reputation; that is to say, other animals are clearly or dimly aware that the crab is a voracious and combative creature. How useful to the crab, then, to have its appearance cloaked by a growth of innocent seaweed, or sponge, or zoophyte. It will enable the creature to sneak upon its victims or to escape the attention of its own enemies.
If a narrow-beaked crab is cleaned artificially it will proceed to clothe itself again, the habit has become instinctive; and it must be admitted that while a particular crab prefers a particular kind of seaweed for its dress, it will cover itself with unsuitable and even conspicuous material, such as pieces of coloured cloth, if nothing better is available. The disguise differs greatly, for one crab is masked by a brightly coloured and unpalatable sponge densely packed with flinty needles; another cuts off the tunic of a sea-squirt and throws it over its shoulders; another trundles about a bivalve sh.e.l.l. The facts recall the familiar case of the hermit-crab, which protects its soft tail by tucking it into the empty sh.e.l.l of a periwinkle or a whelk or some other sea-snail, and that case leads on to the elaboration known as commensalism, where the hermit-crab fixes sea-anemones on the back of its borrowed house. The advantage here is beyond that of masking, for the sea-anemone can sting, which is a useful quality in a partner. That this second advantage may become the main one is evident in several cases where the sea-anemone is borne, just like a weapon, on each of the crustacean"s great claws. Moreover, as the term commensalism (eating at the same table) suggests, the partnership is mutuallybeneficial. For the sea-anemone is carried about by the hermit-crab, and it doubtless gets its share of crumbs from its partner"s frequent meals. There is a very interesting sidelight on the mutual benefit in the case of a dislodged sea-anemone which sulked for a while and then waited in a state of preparedness until a hermit-crab pa.s.sed by and touched it. Whereupon the sea-anemone gripped and slowly worked itself up on to the back of the sh.e.l.l.
-- 6.
Other Kinds of Elusiveness.
There are various kinds of disguise which are not readily cla.s.sified. A troop of cuttlefish swimming in the sea is a beautiful sight. They keep time with one another in their movements and they show the same change of colour almost at the same moment. They are suddenly attacked, however, by a small shark, and then comes a simultaneous discharge of sepia from their ink-bags. There are clouds of ink in the clear water, for, as Professor Hickson puts it, the cuttlefishes have thrown dust in the eyes of their enemies. One can see a newborn cuttlefish do this a minute after it escapes from the egg.
Very beautiful is the way in which many birds, like our common chaffinch, disguise the outside of their nest with moss and lichen and other trifles felted together, so that the cradle is as inconspicuous as possible. There seems to be a touch of art in fastening pieces of spider"s web on the outside of a nest!
How curious is the case of the tree-sloth of South American forests, that walks slowly, back downwards, along the undersides of the branches, hanging on by its long, curved fingers and toes. It is a nocturnal animal, and therefore not in special danger, but when resting during the day it is almost invisible because its s.h.a.ggy hair is so like certain lichens and other growths on the branches. But the protective resemblance is enhanced by the presence of a green alga, which actually lives on the surface of the sloth"s hairs--an alga like the one that makes tree-stems and gate-posts green in damp weather.
There is no commoner sight in the early summer than the cuckoo-spit on the gra.s.ses and herbage by the wayside. It is conspicuous and yet it is said to be left severely alone by almost all creatures. In some way it must be a disguise. It is a sort of soap made by the activity of small frog-hoppers while they are still in the wingless larval stage, before they begin to hop. The insect pierces with its sharp mouth-parts the skin of the plant and sucks in sweet sap which by and by overflows over its body. It works its body up and down many times, whipping in air, which mixes with the sugary sap, reminding one of how "whipped egg" is made. But along with the sugary sap and the air, there is a little ferment from the food-ca.n.a.l and a little wax from glands on the skin, and the four things mixed together make a kind of soap which lasts through the heat of the day.
There are many other modes of disguise besides those which we have been able to ill.u.s.trate. Indeed, the biggest fact is that there are so many, for it brings us back to the idea that life is not an easy business. It is true, as Walt Whitman says, that animals do not sweat and whine about their condition; perhaps it is true, as he says, that not one is unhappy over the whole earth. But there is another truth, that this world is not a place for the unlit lamp and the ungirt loin, and that when a creature has not armour or weapons or cleverness it must find some path of safety or go back. One of these paths of safety is disguise, and we have ill.u.s.trated its evolution.
V.
THE ASCENT OF MAN.
-- 1.
No one thinks less of Sir Isaac Newton because he was born as a very puny infant, and no one should think less of the human race because it sprang from a stock of arboreal mammals. There is no doubt as to man"s apartness from the rest of creation when he is seen at his best--"a little lower than the angels, crowned with glory and honour." "What a piece of work is a man! How n.o.ble in reason! How infinite in faculty! in form and moving how express and admirable! in action how like an angel! in apprehension so like a G.o.d." Nevertheless, all the facts point to his affiliation to the stock to which monkeys and apes also belong. Not, indeed, that man is descended from any living ape or monkey; it is rather that he and they have sprung from a common ancestry--are branches of the same stem. This conclusion is so momentous that the reasons for accepting it must be carefully considered. They were expounded with masterly skill in Darwin"s Descent of Man in 1871--a book which was but an expansion of a chapter in The Origin of Species (1859).
Anatomical Proof of Man"s Relationship with a Simian Stock The anatomical structure of man is closely similar to that of the anthropoid apes--the gorilla, the orang, the chimpanzee, and the gibbon. Bone for bone, muscle for muscle, blood-vessel for blood-vessel, nerve for nerve, man and ape agree. As the conservative anatomist, Sir Richard Owen, said, there is between them "an all-pervading similitude of structure." Differences, of course, there are, but they are not momentous except man"s big brain, which may be three times as heavy as that of a gorilla. The average human brain weighs about 48 ounces; the gorilla brain does not exceed 20 ounces at its best. The capacity of the human skull is never less than 55 cubic inches; in the orang and the chimpanzee the figures are 26 and 27-1/2 respectively. We are not suggesting that the most distinctive features of man are such as can be measured and weighed, but it is important to notice that the main seat of his mental powers is physically far ahead of that of the highest of the anthropoid apes.
Man alone is thoroughly erect after his infancy is past; his head weighted with the heavy brain does not droop forward as the ape"s does; with his erect att.i.tude there is perhaps to be a.s.sociated his more highly developed vocal organs. Compared with an anthropoid ape, man has a bigger and more upright forehead, a less protrusive face region, smaller cheek-bones and eyebrow ridges, and more uniform teeth. He is almost unique in having a chin. Man plants the sole of his foot flat on the ground, his big toe is usually in a line with the other toes, and he has a better heel than any monkey has. The change in the shape of the head is to be thought of in connection with the enlargement of the brain, and also in connection with the natural reduction of the muzzle region when the hand was freed from being an organ of support and became suited for grasping the food and conveying it to the mouth.
Everyone is familiar in man"s clothing with traces of the past persisting in the present, though their use has long since disappeared. There are b.u.t.tons on the back of the waist of the morning coat to which the tails of the coat used to be fastened up, and there are b.u.t.tons, occasionally with b.u.t.tonholes, at the wrist which were once useful in turning up the sleeve. The same is true of man"s body, which is a veritable museum of relics. Some anatomists have made out a list of over a hundred of these vestigial structures, and though this number is perhaps too high, there is no doubt that the list is long. In the inner upper corner of the eye there is a minute tag--but larger in some races than in others--which is the last dwindling relic of the third eyelid, used in cleaning the front of the eye, which most mammals possess in a large and well-developed form. It can be easily seen, for instance, in ox and rabbit. In man and in monkeys it has become a useless vestige, and the dwindling must be a.s.sociated with the fact that the upper eyelid is much more mobile in man and monkeys than in the other mammals. The vestigial third eyelid in man is enough of itself to prove his relationship with the mammals, but it is only one example out of many. Some of these are discussed in the article dealing with the human body, but we may mention the vestigial muscles going to the ear-trumpet, man"s dwindling counterpart of the skin-twitching muscle which we see a horse use when he jerks a fly off his flanks, and the short tail which in the seven-weeks-old human embryo is actually longer than the leg. Without committing ourselves to a belief in the entire uselessness of the vermiform appendix, which grows out as a blind alley at the junction of the small intestine with the large, we are safe in saying that it is a dwindling structure--the remains of a blind gut which must have been capacious and useful in ancestral forms. In some mammals, like the rabbit, the blind gut is the bulkiest structure in the body, and bears the vermiform appendix at its far end. In man the appendix alone is left, and it tells its tale. It is interesting to notice that it is usually longer in the orang than in man, and that it is very variable, as dwindling structures tend to be. One of the unpleasant expressions of this variability is the liability to go wrong: hence appendicitis. Now these vestigial structures are, as Darwin said, like the unsounded, i.e. functionless, letters in words, such as the oin "leopard," the b in "doubt," the g in "reign." They are of no use, but they tell us something of the history of the words. So do man"s vestigial structures reveal his pedigree. They must have an historical or evolutionary significance. No other interpretation is possible.
[Ill.u.s.tration: Photo: New York Zoological Park.
CHIMPANZEE, SITTING.
The head shows certain facial characteristics, e.g. the beetling eyebrow ridges, which were marked in the Neanderthal race of men. Note the shortening of the thumb and the enlargement of the big toe.]
[Ill.u.s.tration: Photo: New York Zoological Park.
CHIMPANZEE, ILl.u.s.tRATING WALKING POWERS.
Note the great length of the arms and the relative shortness of the legs.]
[Ill.u.s.tration: SURFACE VIEW OF THE BRAINS OF MAN (1) AND CHIMPANZEE (2) The human brain is much larger and heavier, more dome-like, and with much more numerous and complicated convolutions.]
[Ill.u.s.tration: Photo: New York Zoological Park.
SIDE-VIEW OF CHIMPANZEE"S HEAD.
(Compare with opposite picture.)]
[Ill.u.s.tration: After a model by J. H. McGregor.
PROFILE VIEW OF HEAD OF PITHECANTHROPUS, THE JAVA APE MAN, RECONSTRUCTED FROM THE SKULL-CAP.]
[Ill.u.s.tration: THE FLIPPER OF A WHALE AND THE HAND OF A MAN.
In the bones and in their arrangement there is a close resemblance in the two cases, yet the outcome is very different. The multiplication of finger joints in the whale is a striking feature.]
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."
-- 2.
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 vertebr- 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.
-- 3.
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).