3. THE DEEP-SEA FISH CHIASMODON NIGER IS FAMOUS FOR ITS VORACITY. It sometimes manages to swallow a fish larger than itself, which causes an extraordinary protrusion of the stomach.
4. DEEP-SEA FISHES. Two of them--Melanocetus murrayi and Melanocetus indicus--are related to the Angler of British coasts, but adapted to life in the great abysses. They are very dark in colour, and delicately built; they possess well-developed luminous organs. The third form is called Chauliodus, a predatory animal with large gape and formidable teeth.]
[Ill.u.s.tration: FLINTY SKELETON OF VENUS FLOWER BASKET (EUPLECTELLA), A j.a.pANESE DEEP-SEA SPONGE]
[Ill.u.s.tration: EGG DEPOSITORY OF Semotilus Atromaculatus.
In the building of this egg depository, the male fish takes stones from the bottom of the stream, gripping them in his mouth, and heaps them up into the dam. In the egg depository he arranges the stones so that when the eggs are deposited in the interstices they are thoroughly protected, and cannot be washed down-stream.
1, dam of stones; 2, egg depository; 3, hillock of sand. The arrow shows the direction of the stream. Upper fish, male; lower, female.]
The "floating sea-meadows," as Sir John Murray called them, are always receiving contributions from insh.o.r.e waters, where the conditions are favourable for the prolific multiplication of unicellular Alg-, and there is also a certain amount of non-living sea-dust always being swept out from the seaweed and sea-gra.s.s area.
Swimmers and Drifters.
The animals of the open sea are conveniently divided into the active swimmers (Nekton) and the more pa.s.sive drifters (Plankton). The swimmers include whales great and small, such birds as the storm petrel, the fish-eating turtles and sea-snakes, such fishes as mackerel and herring, the winged snails or sea-b.u.t.terflies on which whalebone whales largely feed, some of the active cuttles or squids, various open-sea prawns and their relatives, some worms like the transparent arrow-worm, and such active Protozoa as Noctiluca, whose luminescence makes the waves sparkle in the short summer darkness. Very striking as an instance of the insurgence of life are the sea-skimmers (Halobatid-), wingless insects related to the water-measurers in the ditch. They are found hundreds of miles from land, skimming on the surface of the open sea, and diving in stormy weather. They feed on floating dead animals.
The drifters or easygoing swimmers--for there is no hard and fast line--are represented, for instance, by the flinty-sh.e.l.led Radiolarians and certain of the chalk-forming animals (Globigerinid Foraminifera); by jellyfishes, swimming-bells, and Portuguese men-of-war; by the comb-bearers or Ctenoph.o.r.es; by legions of minute Crustaceans; by strange animals called Salps, related to the sedentary sea-squirts; and by some sluggish fishes like globe-fishes, which often float idly on the surface.
Open-sea animals tend to be delicately built, with a specific gravity near that of the sea-water, with adaptations, such as projecting filaments, which help flotation, and with capacities of rising and sinking according to the surrounding conditions. Many of them are luminescent, and many of them are very inconspicuous in the water owing to their transparency or their bluish colour. In both cases the significance is obscure.
Hunger and Love.
Hunger is often very much in evidence in the open sea, especially in areas where the Plankton is poor. For there is great diversity in this respect, most of the Mediterranean, for instance, having a scanty Plankton as compared with the North Sea. In the South Pacific, west of Patagonia, there is said to be an immense "sea desert" where there is little Plankton, and therefore little in the way of fishes. The success of fisheries in the North, e.g. on the Atlantic cod-banks, is due to the richness of the floating sea-meadows and the abundance of the smaller const.i.tuents of the animal Plankton.
Hunger is plain enough when the Baleen Whale rushes through the water with open jaws, engulfing in the huge cavern of its mouth, where the pendent whalebone plates form a huge sieve, incalculable millions of small fry.
But there is love as well as hunger in the open sea. The maternal care exhibited by the whale reaches a very high level, and the delicate sh.e.l.l of the female Paper Nautilus or Argonaut, in which the eggs and the young ones are sheltered, may well be described as "the most beautiful cradle in the world."
Besides the permanent inhabitants of the open sea, there are the larval stages of many sh.o.r.e-animals which are there only for a short time. For there is an interesting give and take between the sh.o.r.e-haunt and the open sea. From the sh.o.r.e come nutritive contributions and minute organisms which multiply quickly in the open waters. But not less important is the fact that the open waters afford a safe cradle or nursery for many a delicate larva, e.g. of crab and starfish, acorn-sh.e.l.l and sea-urchin, which could not survive for a day in the rough-and-tumble conditions of the sh.o.r.e and the shallow water. After undergoing radical changes and gaining strength, the young creatures return to the sh.o.r.e in various ways.
III. THE DEEP SEA.
Very different from all the other haunts are the depths of the sea, including the floor of the abysses and the zones of water near the bottom. This haunt, forever unseen, occupies more than a third of the earth"s surface, and it is thickly peopled. It came into emphatic notice in connection with the mending of telegraph cables, but the results of the Challenger expedition (1873-6) gave the first impressive picture of what was practically a new world.
Physical Conditions.
The average depth of the ocean is about two and a half miles; therefore, since many parts are relatively shallow, there must be enormous depths. A few of these, technically called "deeps," are about six miles deep, in which Mount Everest would be engulfed. There is enormous pressure in such depths; even at 2,500 fathoms it is two and a half tons on the square inch. The temperature is on and off the freezing-point of fresh water (28-34 Fahr.), due to the continual sinking down of cold water from the Poles, especially from the South. Apart from the fitful gleams of luminescent animals, there is utter darkness in the deep waters. The rays of sunlight are practically extinguished at 250 fathoms, though very sensitive bromogelatine plates exposed at 500 fathoms have shown faint indications even at that depth. It is a world of absolute calm and silence, and there is no scenery on the floor. A deep, cold, dark, silent, monotonous world!
Biological Conditions.
While some parts of the floor of the abysses are more thickly peopled than others, there is no depth limit to the distribution of life. Wherever the long arm of the dredge has reached, animals have been found, e.g. Protozoa, sponges, corals, worms, starfishes, sea-urchins, sea-lilies, crustaceans, lamp-sh.e.l.ls, molluscs, ascidians, and fishes--a very representative fauna. In the absence of light there can be no chlorophyll-possessing plants, and as the animals cannot all be eating one another there must be an extraneous source of food-supply. This is found in the sinking down of minute organisms which are killed on the surface by changes of temperature and other causes. What is left of them, before or after being swallowed, and of sea-dust and mineral particles of various kinds forms the diversified "ooze" of the sea-floor, a soft muddy precipitate, which is said to have in places the consistence of b.u.t.ter in summer weather.
There seems to be no bacteria in the abysses, so there can be no rotting. Everything that sinks down, even the huge carcase of a whale, must be nibbled away by hungry animals and digested, or else, in the case of most bones, slowly dissolved away. Of the whale there are left only the ear-bones, of the shark his teeth.
Adaptations to Deep-sea Life.
In adaptation to the great pressure the bodies of deep-sea animals are usually very permeable, so that the water gets through and through them, as in the case of Venus" Flower Basket, a flinty sponge which a child"s finger would shiver. But when the pressure inside is the same as that outside nothing happens. In adaptation to the treacherous ooze, so apt to smother, many of the active deep-sea animals have very long, stilt-like legs, and many of the sedentary types are lifted into safety on the end of long stalks which have their bases embedded in the mud. In adaptation to the darkness, in which there is only luminescence that eyes could use, there is a great development of tactility. The interesting problem of luminescence will be discussed elsewhere.
As to the origin of the deep-sea fauna, there seems no doubt that it has arisen by many contributions from the various sh.o.r.e-haunts. Following the down-drifting food, many sh.o.r.e-animals have in the course of many generations reached the world of eternal night and winter, and become adapted to its strange conditions. For the animals of the deep-sea are as fit, beautiful, and vigorous as those elsewhere. There are no slums in Nature.
[Ill.u.s.tration: THE BITTERLING (Rhodeus Amarus).
A Continental fish which lays its eggs by means of a long ovipositor inside the freshwater mussel. The eggs develop inside the mollusc"s gill-plates.]
[Ill.u.s.tration: Photo: W. S. Berridge.
WOOLLY OPOSSUM CARRYING HER FAMILY.
One of the young ones is clinging to its mother and has its long prehensile tail coiled round hers.]
[Ill.u.s.tration: SURINAM TOAD (Pipa Americana) WITH YOUNG ONES HATCHING OUT OF LITTLE POCKETS ON HER BACK]
[Ill.u.s.tration: STORM PETREL OR MOTHER CAREY"S CHICKEN.
(Procellaria Pelagica).
This characteristic bird of the open sea does not come to land at all except to nest. It is the smallest web-footed bird, about four inches long. The legs are long and often touch the water as the bird flies. The storm petrel is at home in the Atlantic, and often nests on islands off the west coast of Britain.]
IV. THE FRESH WATERS.
Of the whole earth"s surface the freshwaters form a very small fraction, about a hundredth, but they make up for their smallness by their variety. We think of deep lake and shallow pond, of the great river and the purling brook, of lagoon and swamp, and more besides. There is a striking resemblance in the animal population of widely separated freshwater basins: and this is partly because birds carry many small creatures on their muddy feet from one water-shed to another; partly because some of the freshwater animals are descended from types which make their way from the sea and the seash.o.r.e through estuaries and marshes, and only certain kinds of const.i.tution could survive the migration; and partly because some lakes are landlocked dwindling relics of ancient seas, and similar forms again would survive the change.
A typical a.s.semblage of freshwater animals would include many Protozoa, like Amoeb- and the Bell-Animalcules, a representative of one family of sponges (Spongillid-), the common Hydra, many unsegmented worms (notably Planarians and Nematodes), many Annelids related to the earthworms, many crustaceans, insects, and mites, many bivalves and snails, various fishes, a newt or two, perhaps a little mud-turtle or in warm countries a huge Crocodilian, various interesting birds like the water-ouzel or dipper, and mammals like the water-vole and the water-shrew.
Freshwater animals have to face certain difficulties, the greatest of which are drought, frost, and being washed away in times of flood. There is no more interesting study in the world than an inquiry into the adaptations by which freshwater animals overcome the difficulties of the situation. We cannot give more than a few ill.u.s.trations.
(1) Drought is circ.u.mvented by the capacity that many freshwater animals have of lying low and saying nothing. Thus the African mudfish may spend half the year encased in the mud, and many minute crustaceans can survive being dried up for years. (2) Escape from the danger of being frozen hard in the pool is largely due to the almost unique property of water that it expands as it approaches the freezing-point. Thus the colder water rises to the surface and forms or adds to the protecting blanket of ice. The warmer water remains unfrozen at the bottom, and the animals live on. (3) The risk of being washed away, e.g. to the sea, is lessened by all sorts of gripping, grappling, and anchoring structures, and by shortening the juvenile stages when the risks are greatest.
V. THE DRY LAND.
Over and over again in the history of animal life there have been attempts to get out of the water on to terra firma, and many of these have been successful, notably those made (1) by worms, (2) by air-breathing Arthropods, and (3) by amphibians.
In thinking of the conquest of the dry land by animals, we must recognise the indispensable role of plants in preparing the way. The dry ground would have proved too inhospitable had not terrestrial plants begun to establish themselves, affording food, shelter, and humidity. There had to be plants before there could be earthworms, which feed on decaying leaves and the like, but how soon was the debt repaid when the earthworms began their worldwide task of forming vegetable mould, opening up the earth with their burrows, circulating the soil by means of their castings, and bruising the particles in their gizzard--certainly the most important mill in the world.
Another important idea is that littoral haunts, both on the seash.o.r.e and in the freshwaters, afforded the necessary apprenticeship and transitional experience for the more strenuous life on dry land. Much that was perfected on land had its beginnings on the sh.o.r.e. Let us inquire, however, what the pa.s.sage from water to dry land actually implied. This has been briefly discussed in a previous article (on Evolution), but the subject is one of great interest and importance.
Difficulties and Results of the Transition from Water to Land.
Leaving the water for dry land implied a loss in freedom of movement, for the terrestrial animal is primarily restricted to the surface of the earth. Thus it became essential that movements should be very rapid and very precise, needs with which we may a.s.sociate the acquisition of fine cross-striped, quickly contracting muscles, and also, in time, their multiplication into very numerous separate engines. We exercise fifty-four muscles in the half-second that elapses between raising the heel of our foot in walking and planting it firmly on the ground again. Moreover, the need for rapid precisely controlled movements implied an improved nervous system, for the brain was a movement-controlling organ for ages before it did much in the way of thinking. The transition to terra firma also involved a greater compactness of body, so that there should not be too great friction on the surface. An animal like the jellyfish is unthinkable on land, and the elongated bodies of some land animals like centipedes and snakes are specially adapted so that they do not "sprawl." They are exceptions that prove the rule.
Getting on to dry land meant entering a kingdom where the differences between day and night, between summer and winter are more felt than in the sea. This made it advantageous to have protections against evaporation and loss of heat and other such dangers. Hence a variety of ways in which the surface of the body acquired a thickened skin, or a dead cuticle, or a sh.e.l.l, or a growth of hair, and so forth. In many cases there is an increase of the protection before the winter sets in, e.g. by growing thicker fur or by acc.u.mulating a layer of fat below the skin.
But the thickening or protection of the skin involved a partial or total loss of the skin as a respiratory surface. There is more oxygen available on dry land than in the water, but it is not so readily captured. Thus we see the importance of moist internal surfaces for capturing the oxygen which has been drawn into the interior of the body into some sort of lung. A unique solution was offered by Tracheate Arthropods, such as Peripatus, Centipedes, Millipedes, and Insects, where the air is carried to every hole and corner of the body by a ramifying system of air-tubes or trache-. In most animals the blood goes to the air, in insects the air goes to the blood. In the Robber-Crab, which has migrated from the sh.o.r.e inland, the dry air is absorbed by vascular tufts growing under the shelter of the gill-cover.
The problem of disposing of eggs or young ones is obviously much more difficult on land than in the water. For the water offers an immediate cradle, whereas on the dry land there were many dangers, e.g. of drought, extremes of temperature, and hungry sharp-eyed enemies, which had to be circ.u.mvented. So we find all manner of ways in which land animals hide their eggs or their young ones in holes and nests, on herbs and on trees. Some carry their young ones about after they are born, like the Surinam toad and the kangaroo, while others have prolonged the period of ante-natal life during which the young ones develop in safety within their mother, and in very intimate partnership with her in the case of the placental mammals. It is very interesting to find that the pioneer animal called Peripatus, which bridges the gap between worms and insects, carries its young for almost a year before birth.
Enough has been said to show that the successive conquests of the dry land had great evolutionary results. It is hardly too much to say that the invasion which the Amphibians led was the beginning of better brains, more controlled activities, and higher expressions of family life.
[Ill.u.s.tration: ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN SEA.
It may have a spread of wing of over 11 feet from tip to tip. It is famous for its extraordinary power of "sailing" round the ship without any apparent strokes of its wings.]
VI. THE AIR.
There are no animals thoroughly aerial, but many insects spend much of their adult life in the free air, and the swift hardly pauses in its flight from dawn to dusk of the long summer day, alighting only for brief moments at the nest to deliver insects to the young. All the active life of bats certainly deserves to be called aerial.
The air was the last haunt of life to be conquered, and it is interesting to inquire what the conquest implied. (1) It meant transcending the radical difficulty of terrestrial life which confines the creatures of the dry land to moving on one plane, the surface of the earth. But the power of flight brought its possessors back to the universal freedom of movement which water animals enjoy. When we watch a sparrow rise into the air just as the cat has completed her stealthy stalking, we see that flight implies an enormous increase of safety. (2) The power of flight also opened up new possibilities of following the prey, of exploring new territories, of prospecting for water. (3) Of great importance too was the practicability of placing the eggs and the young, perhaps in a nest, in some place inaccessible to most enemies. When one thinks of it, the rooks" nests swaying on the tree-tops express the climax of a brilliant experiment. (4) The crowning advantage was the possibility of migrating, of conquering time (by circ.u.mventing the arid summer and the severe winter) and of conquering s.p.a.ce (by pa.s.sing quickly from one country to another and sometimes almost girdling the globe). There are not many acquisitions that have meant more to their possessors than the power of flight. It was a key opening the doors of a new freedom.
The problem of flight, as has been said in a previous chapter, has been solved four times, and the solution has been different in each case. The four solutions are those offered by insects, extinct Pterodactyls, birds, and bats. Moreover, as has been pointed out, there have been numerous attempts at flight which remain glorious failures, notably the flying fishes, which take a great leap and hold their pectoral fins taut; the Flying Tree-Toad, whose webbed fingers and toes form a parachute; the Flying Lizard (Draco volans), which has its skin pushed out on five or six greatly elongated mobile ribs; and various "flying" mammals, e.g. Flying Phalangers and Flying Squirrels, which take great swooping leaps from tree to tree.
The wings of an insect are hollow flattened sacs which grow out from the upper parts of the sides of the second and third rings of the region called the thorax. They are worked by powerful muscles, and are supported, like a fan, by ribs of chitin, which may be accompanied by air-tubes, blood-channels, and nerves. The insect"s body is lightly built and very perfectly aerated, and the principle of the insect"s flight is the extremely rapid striking of the air by means of the lightly built elastic wings. Many an insect has over two hundred strokes of its wings in one second. Hence, in many cases, the familiar hum, comparable on a small scale to that produced by the rapidly revolving blades of an aeroplane"s propeller. For a short distance a bee can outfly a pigeon, but few insects can fly far, and they are easily blown away or blown back by the wind. Dragon-flies and bees may be cited as examples of insects that often fly for two or three miles. But this is exceptional, and the usual shortness of insect flight is an important fact for man since it limits the range of insects like house-flies and mosquitoes which are vehicles of typhoid fever and malaria respectively. The most primitive insects (spring-tails and bristle-tails) show no trace of wings, while fleas and lice have become secondarily wingless. It is interesting to notice that some insects only fly once in their lifetime, namely, in connection with mating. The evolution of the insect"s wing remains quite obscure, but it is probable that insects could run, leap, and parachute before they could actually fly.
The extinct Flying Dragons or Pterodactyls had their golden age in the Cretaceous era, after which they disappeared, leaving no descendants. A fold of skin was spread out from the sides of the body by the enormously elongated outermost finger (usually regarded as corresponding to our little finger); it was continued to the hind-legs and thence to the tail.
It is unlikely that the Pterodactyls could fly far, for they have at most a weak keel on their breast-bone; on the other hand, some of them show a marked fusion of dorsal vertebr-, which, as in flying birds, must have served as a firm fulcrum for the stroke of the wings. The quaint creatures varied from the size of a sparrow up to a magnificent spread of 15-20 feet from tip to tip of the wings. They were the largest of all flying creatures.
The bird"s solution of the problem of flight, which will be discussed separately, is centred in the feather, which forms a coherent vane for striking the air. In Pterodactyl and bat the wing is a web-wing or patagium, and a small web is to be seen on the front side of the bird"s wing. But the bird"s patagium is unimportant, and the bird"s wing is on an evolutionary tack of its own--a fore-limb transformed for bearing the feathers of flight. Feathers are in a general way comparable to the scales of reptiles, but only in a general way, and no transition stage is known between the two. Birds evolved from a bipedal Dinosaur stock, as has been noticed already, and it is highly probable that they began their ascent by taking running leaps along the ground, flapping their scaly fore-limbs, and balancing themselves in kangaroo-like fashion with an extended tail. A second chapter was probably an arboreal apprenticeship, during which they made a fine art of parachuting--a persistence of which is to be seen in the pigeon "gliding" from the dovecot to the ground. It is in birds that the mastery of the air reaches its climax, and the mysterious "sailing" of the albatross and the vulture is surely the most remarkable locomotor triumph that has ever been achieved. Without any apparent stroke of the wings, the bird sails for half an hour at a time with the wind and against the wind, around the ship and in majestic spirals in the sky, probably taking advantage of currents of air of different velocities, and continually changing energy of position into energy of motion as it sinks, and energy of motion into energy of position as it rises. It is interesting to know that some dragon-flies are also able to "sail."
The web-wing of bats involves much more than the fore-arm. The double fold of skin begins on the side of the neck, pa.s.ses along the front of the arm, skips the thumb, and is continued over the elongated palm-bones and fingers to the sides of the body again, and to the hind-legs, and to the tail if there is a tail. It is interesting to find that the bones of the bat"s skeleton tend to be lightly built as in birds, that the breast-bone has likewise a keel for the better insertion of the pectoral muscles, and that there is a solidifying of the vertebr- of the back, affording as in birds a firm basis for the wing action. Such similar adaptations to similar needs, occurring in animals not nearly related to one another, are called "convergences," and form a very interesting study. In addition to adaptations which the bat shares with the flying bird, it has many of its own. There are so many nerve-endings on the wing, and often also on special skin-leaves about the ears and nose, that the bat flying in the dusk does not knock against branches or other obstacles. Some say that it is helped by the echoes of its high-pitched voice, but there is no doubt as to its exquisite tactility. That it usually produces only a single young one at a time is a clear adaptation to flight, and similarly the sharp, mountain-top-like cusps on the back teeth are adapted in insectivorous bats for crunching insects.
Whether we think of the triumphant flight of birds, reaching a climax in migration, or of the marvel that a creature of the earth--as a mammal essentially is--should evolve such a mastery of the air as we see in bats, or even of the repeated but splendid failures which parachuting animals ill.u.s.trate, we gain an impression of the insurgence of living creatures in their characteristic endeavour after fuller well-being.
We have said enough to show how well adapted many animals are to meet the particular difficulties of the haunt which they tenant. But difficulties and limitations are ever arising afresh, and so one fitness follows on another. It is natural, therefore, to pa.s.s to the frequent occurrence of protective resemblance, camouflage, and mimicry--the subject of the next article.
BIBLIOGRAPHY.
ELMHIRST, R., Animals of the Sh.o.r.e. FLATTELY AND WALTON, The Biology of the Sh.o.r.e (1921). FURNEAUX, Life of Ponds and Streams. HICKSON, S. J., Story of Life in the Seas and Fauna of the Deep Sea. JOHNSTONE, J., Life in the Sea (Cambridge Manual of Science). MIALL, L. C., Aquatic Insects. MURRAY, SIR JOHN, The Ocean (Home University Library). MURRAY, SIR JOHN AND HJORT, DR. J., The Depths of the Ocean. NEWBIGIN, M. I., Life by the Sea Sh.o.r.e. PYCRAFT, W. P., History of Birds. SCHARFF, R. F., History of the European Fauna (Contemp. Sci. Series). THOMSON, J. ARTHUR, The Wonder of Life (1914) and The Haunts of Life (1921).
IV.
THE STRUGGLE FOR EXISTENCE.
ANIMAL AND BIRD MIMICRY AND DISGUISE.
-- 1.
For every animal one discovers when observing carefully, there must be ten unseen. This is partly because many animals burrow in the ground or get in underneath things and into dark corners, being what is called cryptozoic or elusive. But it is partly because many animals put on disguise or have in some way acquired a garment of invisibility. This is very common among animals, and it occurs in many forms and degrees. The reason why it is so common is because the struggle for existence is often very keen, and the reasons why the struggle for existence is keen are four. First, there is the tendency to over-population in many animals, especially those of low degree. Second, there is the fact that the scheme of nature involves nutritive chains or successive incarnations, one animal depending upon another for food, and all in the long run on plants; thirdly, every vigorous animal is a bit of a hustler, given to insurgence and sticking out his elbows. There is a fourth great reason for the struggle for existence, namely, the frequent changefulness of the physical environment, which forces animals to answer back or die; but the first three reasons have most to do with the very common a.s.sumption of some sort of disguise. Even when an animal is in no sense a weakling, it may be very advantageous for it to be inconspicuous when it is resting or when it is taking care of its young. Our problem is the evolution of elusiveness, so far at least as that depends on likeness to surroundings, on protective resemblance to other objects, and in its highest reaches on true mimicry.
Colour Permanently Like That of Surroundings.
Many animals living on sandy places have a light-brown colour, as is seen in some lizards and snakes. The green lizard is like the gra.s.s and the green tree-snake is inconspicuous among the branches. The spotted leopard is suited to the interrupted light of the forest, and it is sometimes hard to tell where the jungle ends and the striped tiger begins. There is no better case than the hare or the partridge sitting a few yards off on the ploughed field. Even a donkey grazing in the dusk is much more readily heard than seen.
The experiment has been made of tethering the green variety of Praying Mantis on green herbage, fastening them with silk threads. They escape the notice of birds. The same is true when the brown variety is tethered on withered herbage. But if the green ones are put on brown plants, or the brown ones on green plants, the birds pick them off. Similarly, out of 300 chickens in a field, 240 white or black and therefore conspicuous, 60 spotted and inconspicuous, 24 were soon picked off by crows, but only one of these was spotted. This was not the proportion that there should have been if the mortality had been fortuitous. There is no doubt that it often pays an animal to be like its habitual surroundings, like a little piece of scenery if the animal is not moving. It is safe to say that in process of time wide departures from the safest coloration will be wiped out in the course of Nature"s ceaseless sifting.
But we must not be credulous, and there are three cautions to be borne in mind. (1) An animal may be very like its surroundings without there being any protection implied. The arrow-worms in the sea are as clear as gla.s.s, and so are many open-sea animals. But this is because their tissues are so watery, with a specific gravity near that of the salt water. And the invisibility does not save them, always or often, from being swallowed by larger animals that gather the harvest of the sea. (2) Among the cleverer animals it looks as if the creature sometimes sought out a spot where it was most inconspicuous. A spider may place itself in the middle of a little patch of lichen, where its self-effacement is complete. Perhaps it is more comfortable as well as safer to rest in surroundings the general colour of which is like that of the animal"s body. (3) The fishes that live among the coral-reefs are startling in their brilliant coloration, and there are many different patterns. To explain this it has been suggested that these fishes are so safe among the mazy pa.s.sages and endless nooks of the reefs, that they can well afford to wear any colour that suits their const.i.tution. In some cases this may be true, but naturalists who have put on a diving suit and walked about among the coral have told us that each kind of fish is particularly suited to some particular place, and that some are suited for midday work and others for evening work. Sometimes there is a sort of Box and c.o.x arrangement by which two different fishes utilise the same corner at different times.
[Ill.u.s.tration: THE PRAYING MANTIS (Mantis Religiosa).
A very voracious insect with a quiet, un.o.btrusive appearance. It holds its formidable forelegs as if in the att.i.tude of prayer; its movements are very slow and stealthy; and there is a suggestion of a leaf in the forewing. But there is no reason to credit the creature with conscious guile!]
[Ill.u.s.tration: PROTECTIVE COLORATION: A WINTER SCENE IN NORTH SCANDINAVIA Showing Variable Hare, Willow Grouse, and Arctic Fox, all white in winter and inconspicuous against the snow. But the white dress is also the dress that is physiologically best, for it loses least of the animal heat.]
[Ill.u.s.tration: THE VARIABLE MONITOR (Vara.n.u.s).
The monitors are the largest of existing lizards, the Australian species represented in the photograph attaining a length of four feet. It has a brown colour with yellow spots, and in spite of its size it is not conspicuous against certain backgrounds, such as the bark of a tree.]
-- 2.
Gradual Change of Colour.
The common sh.o.r.e-crab shows many different colours and mottlings, especially when it is young. It may be green or grey, red or brown, and so forth, and it is often in admirable adjustment to the colour of the rock-pool where it is living. Experiments, which require extension, have shown that when the crab has moulted, which it has to do very often when it is young, the colour of the new sh.e.l.l tends to harmonise with the general colour of the rocks and seaweed. How this is brought about, we do not know. The colour does not seem to change till the next moult, and not then unless there is some reason for it. A full-grown sh.o.r.e-crab is well able to look after itself, and it is of interest to notice, therefore, that the variety of coloration is mainly among the small individuals, who have, of course, a much less secure position. It is possible, moreover, that the resemblance to the surroundings admits of more successful hunting, enabling the small crab to take its victim unawares.
Professor Poulton"s experiments with the caterpillars of the small tortoise-sh.e.l.l b.u.t.terfly showed that in black surroundings the pup- tend to be darker, in white surroundings lighter, in gilded boxes golden; and the same is true in other cases. It appears that the surrounding colour affects the caterpillars through the skin during a sensitive period--the twenty hours immediately preceding the last twelve hours of the larval state. The result will tend to make the quiescent pup- less conspicuous during the critical time of metamorphosis. The physiology of this sympathetic colouring remains obscure.
Seasonal Change of Colouring.
The ptarmigan moults three times in the year. Its summer plumage is rather grouselike above, with a good deal of rufous brown; the back becomes much more grey in autumn; almost all the feathers of the winter plumage are white. That is to say, they develop without any pigment and with numerous gas-bubbles in their cells. Now there can be no doubt that this white winter plumage makes the ptarmigan very inconspicuous amidst the snow. Sometimes one comes within a few feet of the crouching bird without seeing it, and this garment of invisibility may save it from the hungry eyes of golden eagles.
Similarly the brown stoat becomes the white ermine, mainly by the growth, of a new suit of white fur, and the same is true of the mountain hare. The ermine is all white except the black tip of its tail; the mountain hare in its winter dress is all white save the black tips of its ears. In some cases, especially in the mountain hare, it seems that individual hairs may turn white, by a loss of pigment, as may occur in man. According to Metchnikoff, the wandering amoeboid cells of the body, called phagocytes, may creep up into the hairs and come back again with microscopic burdens of pigment. The place of the pigment is taken by gas-bubbles, and that is what causes the whiteness. In no animals is there any white pigment; the white colour is like that of snow or foam, it is due to the complete reflection of the light from innumerable minute surfaces of crystals or bubbles.
[Ill.u.s.tration: Photo: W. S. Berridge, F.Z.S.
BANDED KRAIT: A VERY POISONOUS SNAKE WITH ALTERNATING YELLOW AND DARK BANDS.
It is very conspicuous and may serve as an ill.u.s.tration of warning coloration. Perhaps, that is to say, its striking coloration serves as an advertis.e.m.e.nt, impressing other creatures with the fact that the Banded Krait should be left alone. It is very unprofitable for a snake to waste its venom on creatures it does not want.]
[Ill.u.s.tration: Photos: W. S. Berridge, F.Z.S.
THE WARTY CHAMELEON.
The upper photograph shows the Warty Chameleon inflated and conspicuous. At another time, however, with compressed body and adjusted coloration, the animal is very inconspicuous. The lower photograph shows the sudden protrusion of the very long tongue on a fly.]
[Ill.u.s.tration: SEASONAL COLOUR-CHANGE: A SUMMER SCENE IN NORTH SCANDINAVIA.
Showing a brown Variable Hare, Willow Grouse, and Arctic Fox, all inconspicuous in their coloration when seen in their natural surroundings.]
The mountain hare may escape the fox the more readily because its whiteness makes it so inconspicuous against a background of snow; and yet, at other times, we have seen the creature standing out like a target on the dark moorland. So it cuts both ways. The ermine has almost no enemies except the gamekeeper, but its winter whiteness may help it to sneak upon its victims, such as grouse or rabbit, when there is snow upon the ground. In both cases, however, the probability is that the const.i.tutional rhythm which leads to white hair in winter has been fostered and fixed for a reason quite apart from protection. The fact is that for a warm-blooded creature, whether bird or mammal, the physiologically best dress is a white one, for there is less radiation of the precious animal heat from white plumage or white pelage than from any other colour. The quality of warm-bloodedness is a prerogative of birds and mammals, and it means that the body keeps an almost constant temperature, day and night, year in and year out. This is effected by automatic internal adjustments which regulate the supply of heat, chiefly from the muscles, to the loss of heat, chiefly through the skin and from the lungs. The chief importance of this internal heat is that it facilitates the smooth continuance of the chemical processes on which life depends. If the temperature falls, as in hibernating mammals (whose warm-bloodedness is imperfect), the rate of the vital process is slowed down--sometimes dangerously. Thus we see how the white coat helps the life of the creature.
-- 3.