The Outline of Science Part 20

[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 partners.h.i.+p 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 partners.h.i.+p is _mutually_ beneficial. 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

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 Relations.h.i.+p 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 relations.h.i.+p 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 _o_ in "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.]