The Outline of Science Part 6

3. Three-toed horse, Mesohippus, about the size of a sheep. Middle Oligocene, N. America.

4. Three-toed horse, Merychippus, Miocene, N. America. Only one toe reaches the ground on each foot, but the remains of two others are prominent.

5. The first one-toed horse, Pliohippus, about forty inches high at the shoulder. Pliocene, N. America.

6. The modern horse, running on the third digit of each foot.]

There is no warrant for supposing that the flying reptiles or Pterodactyls gave rise to birds, for the two groups are on different lines, and the structure of the wings is entirely different. Thus the long-fingered Pterodactyl wing was a parachute wing, while the secret of the bird's wing has its centre in the feathers. It is highly probable that birds evolved from certain Dinosaurs which had become bipeds, and it is possible that they were for a time swift runners that took "flying jumps" along the ground. Thereafter, perhaps, came a period of arboreal apprenticeship during which there was much gliding from tree to tree before true flight was achieved. It is an interesting fact that the problem of flight has been solved four times among animals--by insects, by Pterodactyls, by birds, and by bats; and that the four solutions are on entirely different lines.

In the Cretaceous period the outstanding events included the waning of giant reptiles, the modernising of the flowering plants, and the multiplication of small mammals. Some of the Permian reptiles, such as the dog-toothed Cynodonts, were extraordinarily mammal-like, and it was probably from among them that definite mammals emerged in the Tria.s.sic. Comparatively little is known of the early Tria.s.sic mammals save that their back-teeth were marked by numerous tubercles on the crown, but they were gaining strength in the late Tria.s.sic when small arboreal insectivores, not very distant from the modern tree-shrews (Tupaia), began to branch out in many directions indicative of the great divisions of modern mammals, such as the clawed mammals, hoofed mammals, and the race of monkeys or Primates. In the Upper Cretaceous there was an exuberant "radiation" of mammals, adaptive to the conquest of all sorts of haunts, and this was vigorously continued in Tertiary times.

There is no difficulty in the fact that the earliest remains of definite mammals in the Tria.s.sic precede the first-known bird in the Jura.s.sic. For although we usually rank mammals as higher than birds (being mammals ourselves, how could we do otherwise?), there are many ways in which birds are pre-eminent, e.g. in skeleton, musculature, integumentary structures, and respiratory system. The fact is that birds and mammals are on two quite different tacks of evolution, not related to one another, save in having a common ancestry in extinct reptiles. Moreover, there is no reason to believe that the Jura.s.sic Arch-opteryx was the first bird in any sense except that it is the first of which we have any record. In any case it is safe to say that birds came to their own before mammals did.

Looking backwards, we may perhaps sum up what is most essential in the Mesozoic era in Professor Schuchert's sentence: "The Mesozoic is the Age of Reptiles, and yet the little mammals and the toothed birds are storing up intelligence and strength to replace the reptiles when the cycads and conifers shall give way to the higher flowering plants."

-- 2.

The Cenozoic or Tertiary Era.

In the Eocene period there was a replacement of the small-brained archaic mammals by big-brained modernised types, and with this must be a.s.sociated the covering of the earth with a garment of gra.s.s and dry pasture. Marshes were replaced by meadows and browsing by grazing mammals. In the spreading meadows an opportunity was also offered for a richer evolution of insects and birds.

During the Oligocene the elevation of the land continued, the climate became much less moist, and the grazing herds extended their range.

The Miocene was the mammalian Golden Age and there were crowning examples of what Osborn calls "adaptive radiation." That is to say, mammals, like the reptiles before them, conquer every haunt of life. There are flying bats, volplaning parachutists, climbers in trees like sloths and squirrels, quickly moving hoofed mammals, burrowers like the moles, freshwater mammals, like duckmole and beaver, sh.o.r.e-frequenting seals and manatees, and open-sea cetaceans, some of which dive far more than full fathoms five. It is important to realise the perennial tendency of animals to conquer every corner and to fill every niche of opportunity, and to notice that this has been done by successive sets of animals in succeeding ages. Most notably the mammals repeat all the experiments of reptiles on a higher turn of the spiral. Thus arises what is called convergence, the superficial resemblance of unrelated types, like whales and fishes, the resemblance being due to the fact that the different types are similarly adapted to similar conditions of life. Professor H. F. Osborn points out that mammals may seek any one of the twelve different habitat-zones, and that in each of these there may be six quite different kinds of food. Living creatures penetrate everywhere like the overflowing waters of a great river in flood.

-- 3.

The Pliocene period was a more strenuous time, with less genial climatic conditions, and with more intense compet.i.tion. Old land bridges were broken and new ones made, and the geographical distribution underwent great changes. Professor R. S. Lull describes the Plioceneas "a period of great unrest." "Many migrations occurred the world over, new compet.i.tions arose, and the weaker stocks began to show the effects of the strenuous life. One momentous event seems to have occurred in the Pliocene, and that was the transformation of the precursor of humanity into man--the culmination of the highest line of evolution."

The Pleistocene period was a time of sifting. There was a continued elevation of the continental ma.s.ses, and Ice Ages set in, relieved by less severe interglacial times when the ice-sheets retreated northwards for a time. Many types, like the mammoth, the woolly rhinoceros, the sabre-toothed tiger, the cave-lion, and the cave-bear, became extinct. Others which formerly had a wide range became restricted to the Far North or were left isolated here and there on the high mountains, like the Snow Mouse, which now occurs on isolated Alpine heights above the snow-line. Perhaps it was during this period that many birds of the Northern Hemisphere learned to evade the winter by the sublime device of migration.

Looking backwards we may quote Professor Schuchert again: "The lands in the Cenozoic began to bloom with more and more flowering plants and grand hardwood forests, the atmosphere is scented with sweet odours, a vast crowd of new kinds of insects appear, and the places of the once dominant reptiles of the lands and seas are taken by the mammals. Out of these struggles there rises a greater intelligence, seen in nearly all of the mammal stocks, but particularly in one, the monkey-ape-man. Brute man appears on the scene with the introduction of the last glacial climate, a most trying time for all things endowed with life, and finally there results the dominance of reasoning man over all his brute a.s.sociates."

In man and human society the story of evolution has its climax.

The Ascent of Man.

Man stands apart from animals in his power of building up general ideas and of using these in the guidance of his behaviour and the control of his conduct. This is essentially wrapped up with his development of language as an instrument of thought. Some animals have words, but man has language (Logos). Some animals show evidence of perceptualinference, but man often gets beyond this to conceptual inference (Reason). Many animals are affectionate and brave, self-forgetful and industrious, but man "thinks the ought," definitely guiding his conduct in the light of ideals, which in turn are wrapped up with the fact that he is "a social person."

Besides his big brain, which may be three times as heavy as that of a gorilla, man has various physical peculiarities. He walks erect, he plants the sole of his foot flat on the ground, he has a chin and a good heel, a big forehead and a non-protrusive face, a relatively uniform set of teeth without conspicuous canines, and a relatively naked body.

[Ill.u.s.tration: DIAGRAM SHOWING SEVEN STAGES IN THE EVOLUTION OF THE FORE-LIMBS AND HIND-LIMBS OF THE ANCESTORS OF THE MODERN HORSE, BEGINNING WITH THE EARLIEST KNOWN PREDECESSORS OF THE HORSE AND CULMINATING WITH THE HORSE OF TO-DAY (After Marsh and Lull.).

1 and 1A, fore-limb and hind-limb of Eohippus; 2 and 2A, Orohippus; 3 and 3A, Mesohippus; 4 and 4A, Hypohippus; 5 and 5A, Merychippus; 6 and 6A, Hipparion; 7 and 7A, the modern horse. Note how the toes shorten and disappear.]

[Ill.u.s.tration: A. Fore-limb of Monkey B. Fore-limb of Whale.

WHAT IS MEANT BY h.o.m.oLOGY? ESSENTIAL SIMILARITY OF ARCHITECTURE, THOUGH THE APPEARANCES MAY BE VERY DIFFERENT.

This is seen in comparing these two fore-limbs, A, of Monkey, B, of Whale. They are as different as possible, yet they show the same bones, e.g. SC, the scapula or shoulder-blade; H, the humerus or upper arm; R and U, the radius and ulna of the fore-arm; CA, the wrist; MC, the palm; and then the fingers.]

But in spite of man's undeniable apartness, there is no doubt as to his solidarity with the rest of creation. There is an "all-pervading similitude of structure," between man and the Anthropoid Apes, though it is certain that it is not from any living form that he took his origin. None of the anatomical distinctions, except the heavy brain, could be called momentous. Man's body is a veritable museum of relics (vestigial structures) inherited from pre-human ancestors. In his everyday bodily life and in some of its disturbances, man's pedigree is often revealed. Even his facial expression, as Darwin showed, is not always human. Some fossil remains bring modern man nearer the anthropoid type.

It is difficult not to admit the ring of truth in 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."

THE EVOLVING SYSTEM OF NATURE.

There is another side of evolution so obvious that it is often overlooked, the tendency to link lives together in vital inter-relations. Thus flowers and their insect visitors are often vitally interlinked in mutual dependence. Many birds feed on berries and distribute the seeds. The tiny freshwater snail is the host of the juvenile stages of the liver-fluke of the sheep. The mosquito is the vehicle of malaria from man to man, and the tse-tse fly spreads sleeping sickness. The freshwater mussel cannot continue its race without the unconscious co-operation of the minnow, and the freshwater fish called the bitterling cannot continue its race without the unconscious co-operation of the mussel. There are numerous mutually beneficial partnerships between different kinds of creatures, and other inter-relations where the benefit is one-sided, as in the case of insects that make galls on plants. There are also among kindred animals many forms of colonies, communities, and societies. Nutritive chains bind long series of animals together, the cod feeding on the whelk, the whelk on the worm, the worm on the organic dust of the sea. There is a system of successive incarnations and matter is continually pa.s.sing from one embodiment to another. These instances must suffice to ill.u.s.trate the central biological idea of the web of life, the interlinked System of Animate Nature. Linn-us spoke of the Systema Natur-, meaning the orderly hierarchy of cla.s.ses, orders, families, genera, and species; but we owe to Darwin in particular some knowledge of a more dynamic Systema Natur-, the network of vital inter-relations. This has become more and more complex as evolution has continued, and man's web is most complex of all. It means making Animate Nature more of a unity; it means an external method of registering steps of progress; it means an evolving set of sieves by which new variations are sifted, and living creatures are kept from slipping down the steep ladder of evolution.

Parasitism.

It sometimes happens that the inter-relation established between one living creature and another works in a retrograde direction. This is the case with many thoroughgoing internal parasites which have sunk into an easygoing kind of life, utterly dependent on their host for food, requiring no exertions, running no risks, and receiving no spur to effort. Thus we see that evolution is not necessarily progressive; everything depends on the conditions in reference to which the living creatures have been evolved. When the conditions are too easygoing, the animal may be thoroughly well adapted to them--as a tapeworm certainly is--but it slips down the rungs of the ladder of evolution.

This is an interesting minor chapter in the story of evolution--the establishment of different kinds of parasites, casual and constant, temporary and lifelong, external hangers-on and internal unpaying boarders, those that live in the food-ca.n.a.l and depend on the host's food and those that inhabit the blood or the tissues and find their food there. It seems clear that ichneumon grubs and the like which hatch inside a caterpillar and eat it alive are not so much parasites as "beasts of prey" working from within.

But there are two sides to this minor chapter: there is the evolution of the parasite, and there is also the evolution of counteractive measures on the part of the host. Thus there is the maintenance of a bodyguard of wandering amoeboid cells, which tackle the microbes invading the body and often succeed in overpowering and digesting them. Thus, again, there is the protective capacity the blood has of making antagonistic substances or "anti-bodies" which counteract poisons, including the poisons which the intruding parasites often make.

THE EVIDENCES OF EVOLUTION--HOW IT CAME ABOUT.

-- 1.

Progress in Evolution.

There has often been slipping back and degeneracy in the course of evolution, but the big fact is that there has been progress. For millions of years Life has been slowly creeping upwards, and if we compare the highest animals--Birds and Mammals--with their predecessors, we must admit that they are more controlled, more masters of their fate, with more mentality. Evolution is on the whole integrative; that is to say, it makes against instability and disorder, and towards harmony and progress. Even in the rise of Birds and Mammals we can discern that the evolutionary process was making towards a fuller embodiment or expression of what Man values most--control, freedom, understanding, and love. The advance of animal life through the ages has been chequered, but on the whole it has been an advance towards increasing fullness, freedom, and fitness of life. In the study of this advance--the central fact of Organic Evolution--there is a.s.suredly much for Man's instruction and much for his encouragement.

Evidences of Evolution.

In all this, it may be said, the fact of evolution has been taken for granted, but what are the evidences? Perhaps it should be frankly answered that the idea of evolution, that the present is the child of the past and the parent of the future, cannot be proved as one may prove the Law of Gravitation. All that can be done is to show that it is a key--a way of looking at things--that fits the facts. There is no lock that it does not open.

But if the facts that the evolution theory vividly interprets be called the evidences of its validity, there is no lack of them. There is historical evidence; and what is more eloquent than the general fact that fishes emerge before amphibians, and these before reptiles, and these before birds, and so on? There are wonderfully complete fossil series, e.g. among cuttlefishes, in which we can almost see evolution in process. The pedigree of horse and elephant and crocodile is in general very convincing, though it is to be confessed that there are other cases in regard to which we have no light. Who can tell, for instance, how Vertebrates arose or from what origin?

There is embryological evidence, for the individual development often reads like an abbreviated recapitulation of the presumed evolution of the race. The mammal's visceral clefts are tell-tale evidence of remote aquatic ancestors, breathing by gills. Something is known in regard to the historical evolution of antlers in bygone ages; the Red Deer of to-day recapitulates at least the general outlines of the history. The individual development of an asymmetrical flat-fish, like a plaice or sole, which rests and swims on one side, tells us plainly that its ancestors were symmetrical fishes.

There is what might be called physiological evidence, for many plants and animals are variable before our eyes, and evolution is going on around us to-day. This is familiarly seen among domesticated animals and cultivated plants, but there is abundant flux in Wild Nature. It need hardly be said that some organisms are very conservative, and that change need not be expected when a position of stable equilibrium has been secured.

There is also anatomical evidence of a most convincing quality. In the fore-limbs of backboned animals, say, the paddle of a turtle, the wing of a bird, the flipper of a whale, the fore-leg of a horse, and the arm of a man; the same essential bones and muscles are used to such diverse results! What could it mean save blood relationship? And as to the two sets of teeth in whalebone whales, which never even cut the gum, is there any alternative but to regard them as relics of useful teeth which ancestral forms possessed? In short, the evolution theory is justified by the way in which it works.

-- 2.

Factors in Evolution.

If it be said "So much for the fact of evolution, but what of the factors?" the answer is not easy. For not only is the problem the greatest of all scientific problems, but the inquiry is still very young. The scientific study of evolution practically dates from the publication of The Origin of Species in 1859.

Heritable novelties or variations often crop up in living creatures, and these form the raw material of evolution. These variations are the outcome of expression of changes in the germ-cells that develop into organisms. But why should there be changes in the const.i.tution of the germ-cells? Perhaps because the living material is very complex and inherently liable to change; perhaps because it is the vehicle of a mult.i.tude of hereditary items among which there are very likely to be reshufflings or rearrangements; perhaps because the germ-cells have very changeful surroundings (the blood, the body-cavity fluid, the sea-water); perhaps because deeply saturating outside influences, such as change of climate and habitat, penetrate through the body to its germ-cells and provoke them to vary. But we must be patient with the wearisome reiteration of "perhaps." Moreover, every many-celled organism reproduced in the usual way, arises from an egg-cell fertilised by a sperm-cell, and the changes involved in and preparatory to this fertilisation may make new permutations and combinations of the living items and hereditary qualities not only possible but necessary. It is something like shuffling a pack of cards, but the cards are living. As to the changes wrought on the body during its lifetime by peculiarities in nurture, habits, and surroundings, these dents or modifications are often very important for the individual, but it does not follow that they are directly important for the race, since it is not certain that they are transmissible.

Given a crop of variations or new departures or mutations, whatever the inborn novelties may be called, we have then to inquire how these are sifted. The sifting, which means the elimination of the relatively less fit variations and the selection of the relatively more fit, effected in many different ways in the course of the struggle for existence. The organism plays its new card in the game of life, and the consequences may determine survival. The relatively less fit to given conditions will tend to be eliminated, while the relatively more fit will tend to survive. If the variations are hereditary and reappear, perhaps increased in amount, generation after generation, and if the process of sifting continue consistently, the result will be the evolution of the species. The sifting process may be helped by various forms of "isolation" which lessen the range of free intercrossing between members of a species, e.g. by geographical barriers. Interbreeding of similar forms tends to make a stable stock; out-breeding among dissimilars tends to promote variability. But for an outline like this it is enough to suggest the general method of organic evolution: Throughout the ages organisms have been making tentatives--new departures of varying magnitude--and these tentatives have been tested. The method is that of testing all things and holding fast that which is good.

BIBLIOGRAPHY.

(The following short list may be useful to readers who desire to have further books recommended to them.) CLODD, Story of Creation: A Plain Account of Evolution. DARWIN, Origin of Species, Descent of Man. DEPERET, Transformation of the Animal World (Internat. Sci. Series). GEDDES AND THOMSON, Evolution (Home University Library). GOODRICH, Evolution (The People's Books). HEADLEY, Life and Evolution. HUTCHINSON, H. NEVILLE, Extinct Monsters (1892). LULL, Organic Evolution. MCCABE, A B C of Evolution. METCALF, Outline of the Theory of Organic Evolution. OSBORN, H. F., The Evolution of Life (1921). THOMSON, Darwinism and Human Life. WALLACE, Darwinism.

III.

ADAPTATIONS TO ENVIRONMENT.

We saw in a previous chapter how the process of evolution led to a mastery of all the haunts of life. But it is necessary to return to these haunts or homes of animals in some detail, so as to understand the peculiar circ.u.mstances of each, and to see how in the course of ages of struggle all sorts of self-preserving and race-continuing adaptations or fitnesses have been wrought out and firmly established. Living creatures have spread over all the earth and in the waters under the earth; some of them have conquered the underground world and others the air. It is possible, however, as has been indicated, to distinguish six great haunts of life, each tenanted by a distinctive fauna, namely, the sh.o.r.e of the sea, the open sea, the depths of the sea, the freshwaters, the dry land, and the air. In the deep sea there are no plants at all; in the air the only plants are floating bacteria, though there is a sense in which a tree is very aerial, and the orchid perched on its branches still more so; in the other four haunts there is a flora as well as a fauna--the two working into one another's hands in interesting and often subtle inter-relations--the subject of a separate study.

I. THE Sh.o.r.e OF THE SEA.

The Seaweed Area.

By the sh.o.r.e of the sea the zoologist means much more than the narrow zone between tide-marks; he means the whole of the relatively shallow, well-illumined, seaweed-growing shelf around the continents and continental islands. Technically, this is called the littoral area, and it is divisible into zones, each with its characteristic population. It may be noted that the green seaweeds are highest up on the sh.o.r.e; the brown ones come next; the beautiful red ones are lowest. All of them have got green chlorophyll, which enables them to utilise the sun's rays in photosynthesis (i.e. building up carbon compounds from air, water, and salts), but in the brown and red seaweeds the green pigment is masked by others. It is maintained by some botanists that these other pigments enable their possessors to make more of the scantier light in the deeper waters. However this may be, we must always think of the sh.o.r.e-haunt as the seaweed-growing area. Directly and indirectly the life of the sh.o.r.e animals is closely wrapped up with the seaweeds, which afford food and foothold, and temper the force of the waves. The minute fragments broken off from seaweeds and from the sea-gra.s.s (a flowering plant called Zostera) form a sort of nutritive sea-dust which is swept slowly down the slope from the sh.o.r.e, to form a very useful deposit in the quietness of deepish water. It is often found in the stomachs of marine animals living a long way offsh.o.r.e.

Conditions of Sh.o.r.e Life.

The littoral area as defined is not a large haunt of life; it occupies only about 9 million square miles, a small fraction of the 197,000,000 of the whole earth's surface. But it is a very long haunt, some 150,000 miles, winding in and out by bay and fiord, estuary and creek. Where deep water comes close to cliffs there may be no sh.o.r.e at all; in other places the relatively shallow water, with seaweeds growing over the bottom, may extend outwards for miles. The nature of the sh.o.r.e varies greatly according to the nature of the rocks, according to what the streams bring down from inland, and according to the jetsam that is brought in by the tides. The sh.o.r.e is a changeful place; there is, in the upper reaches, a striking difference between "tide in" and "tide out"; there are vicissitudes due to storms, to freshwater floods, to wind-blown sand, and to slow changes of level, up and down. The sh.o.r.e is a very crowded haunt, for it is comparatively narrow, and every niche among the rocks may be precious.

[Ill.u.s.tration: AN EIGHT-ARMED CUTTLEFISH OR OCTOPUS ATTACKING A SMALL CRAB.

These molluscs are particularly fond of crustaceans, which they crunch with their parrot's beak-like jaws. Their salivary juice has a paralysing effect on their prey. To one side, below the eye, may be seen the funnel through which water is very forcibly ejected in the process of locomotion.]

[Ill.u.s.tration: A COMMON STARFISH, WHICH HAS LOST THREE ARMS AND IS REGROWING THEM.

The lowest arm is being regrown double..

(After Professor W. C. McIntosh.)]

[Ill.u.s.tration: A PHOTOGRAPH SHOWING A STARFISH (Asterias Forreri) WHICH HAS CAPTURED A LARGE FISH.

The suctorial tube-feet are seen gripping the fish firmly. (After an observation on the Californian coast.)]

[Ill.u.s.tration: Photo: J. J. Ward, F.E.S.

THE PAPER NAUTILUS (ARGONAUTA), AN ANIMAL OF THE OPEN SEA.

The delicate sh.e.l.l is made by the female only, and is used as a shelter for the eggs and young ones. It is secreted by two of the arms, not by the mantle as other mollusc sh.e.l.ls are. It is a single-chambered sh.e.l.l, very different from that of the Pearly Nautilus.]

Keen Struggle for Existence.

It follows that the sh.o.r.e must be the scene of a keen struggle for existence--which includes all the answers-back that living creatures make to environing difficulties and limitations. There is struggle for food, accentuated by the fact that small items tend to be swept away by the outgoing tide or to sink down the slope to deep water. Apart from direct compet.i.tion, e.g. between hungry hermit-crabs, it often involves hard work to get a meal. This is true even of apparently sluggish creatures. Thus the Crumb-of-Bread Sponge, or any other seash.o.r.e sponge, has to lash large quant.i.ties of water through the intricate ca.n.a.l system of its body before it can get a sufficient supply of the microscopic organisms and organic particles on which it feeds. An index of the intensity of the struggle for food is afforded by the nutritive chains which bind animals together. The sh.o.r.e is almost noisy with the conjugation of the verb to eat in its many tenses. One pound of rock-cod requires for its formation ten pounds of whelk; one pound of whelk requires ten pounds of sea-worms; and one pound of worms requires ten pounds of sea-dust. Such is the circulation of matter, ever pa.s.sing from one embodiment or incarnation to another.

Besides struggle for food there is struggle for foothold and for fresh air, struggle against the scouring tide and against the pounding breakers. The risk of dislodgment is often great and the fracture of limbs is a common accident. Of kinds of armour--the sea-urchin's hedgehog-like test, the crab's shard, the limpet's sh.e.l.l--there is great variety, surpa.s.sed only by that of weapons--the sea-anemone's stinging-cells, the sea-urchin's snapping-blades, the hermit-crab's forceps, the grappling tentacles and parrot's-beak jaws of the octopus.

Shifts for a Living.

We get another glimpse of the intensity of the seash.o.r.e struggle for existence in the frequency of "shifts for a living," adaptations of structure or of behaviour which meet frequently recurrent vicissitudes. The starfish is often in the dilemma of losing a limb or its life; by a reflex action it jettisons the captured arm and escapes. And what is lost is gradually regrown. The crab gets its leg broken past all mending; it casts off the leg across a weak breakage plane near the base, and within a preformed bandage which prevents bleeding a new leg is formed in miniature. Such is the adaptive device--more reflex than reflective--which is called self-mutilation or autotomy.

In another part of this book there is a discussion of camouflaging and protective resemblance; how abundantly these are ill.u.s.trated on the sh.o.r.e! But there are other "shifts for a living." Some of the sand-hoppers and their relatives ill.u.s.trate the puzzling phenomenon of "feigning death," becoming suddenly so motionless that they escape the eyes of their enemies. Cuttlefishes, by discharging sepia from their ink-bags, are able to throw dust in the eyes of their enemies. Some undisguised sh.o.r.e-animals, e.g. crabs, are adepts in a hide-and-seek game; some fishes, like the b.u.t.terfish or gunnel, escape between stones where there seemed no opening and are almost uncatchable in their slipperiness. Subtlest of all, perhaps, is the habit some hermit-crabs have of entering into mutually beneficial partnership (commensalism) with sea-anemones, which mask their bearers and also serve as mounted batteries, getting transport as their reward and likewise crumbs from the frequently spread table. But enough has been said to show that the sh.o.r.e-haunt exhibits an extraordinary variety of shifts for a living.

Parental Care on the Sh.o.r.e.

According to Darwin, the struggle for existence, as a big fact in the economy of Animate Nature, includes not only compet.i.tion but all the endeavours which secure the welfare of the offspring, and give them a good send-off in life. So it is without a jolt that we pa.s.s from struggle for food and foothold to parental care. The marine leech called Pontobdella, an interesting greenish warty creature fond of fixing itself to skate, places its egg-coc.o.o.ns in the empty sh.e.l.l of a bivalve mollusc, and guards them for weeks, removing any mud that might injure their development. We have seen a British starfish with its fully-formed young ones creeping about on its body, though the usual mode of development for sh.o.r.e starfishes is that the young ones pa.s.s through a free-swimming larval period in the open water. The father sea-spider carries about the eggs attached to two of his limbs; the father sea-horse puts his mate's eggs into his breast pocket and carries them there in safety until they are hatched; the father stickleback of the sh.o.r.e-pools makes a seaweed nest and guards the eggs which his wives are induced to lay there; the father lumpsucker mounts guard over the bunch of pinkish eggs which his mate has laid in a nook of a rocky sh.o.r.e-pool, and drives off intruders with zest. He also aerates the developing eggs by frequent paddling with his pectoral fins and tail, as the Scots name c.o.c.k-paidle probably suggests. It is interesting that the salient examples of parental care in the sh.o.r.e-haunt are mostly on the male parent's side. But there is maternal virtue as well.

[Ill.u.s.tration: TEN-ARMED CUTTLEFISH OR SQUID IN THE ACT OF CAPTURING A FISH.

The arms bear numerous prehensile suckers, which grip the prey. In the mouth there are strong jaws shaped like a parrot's beak. The cuttlefishes are molluscs and may be regarded as the highest of the backboneless or Invertebrate animals. Many occur near sh.o.r.e, others in the open sea, and others in the great depths.]

[Ill.u.s.tration: GREENLAND WHALE.

Showing the double blowhole or nostrils on the top of the head and the whalebone plates hanging down from the roof of the mouth.]

[Ill.u.s.tration: MINUTE TRANSPARENT EARLY STAGE OF A SEA-CUc.u.mBER.

It swims in the open sea by means of girdles of microscopic cilia shown in the figure. After a period of free swimming and a remarkable metamorphosis, the animal settles down on the floor of the sea in relatively shallow water.]

[Ill.u.s.tration: Photo: British Museum (Natural History).

AN INTRICATE COLONY OF OPEN-SEA ANIMALS (Physophora Hydrostatica) RELATED TO THE PORTUGUESE MAN-OF-WAR.

There is great division of labor in the colony. At the top are floating and swimming "persons"; the long ones below are offensive "persons" bearing batteries of stinging cells; in the middle zone there are nutritive, reproductive, and other "persons." The color of the colony is a fine translucent blue. Swimmers and bathers are often badly stung by this strange animal and its relatives.]

[Ill.u.s.tration: A SCENE IN THE GREAT DEPTHS.

Showing a deep-sea fish of large gape, two feather-stars on the end of long stalks, a "sea-spider" (or Pycnogon) walking on lanky legs on the treacherous ooze, likewise a brittle-star, and some deep-sea corals.]

The fauna of the sh.o.r.e is remarkably representative--from unicellular Protozoa to birds like the oyster-catcher and mammals like the seals. Almost all the great groups of animals have apparently served an apprenticeship in the sh.o.r.e-haunt, and since lessons learned for millions of years sink in and become organically enregistered, it is justifiable to look to the sh.o.r.e as a great school in which were gained racial qualities of endurance, patience, and alertness.

II. THE OPEN SEA.

In great contrast to the narrow, crowded, difficult conditions of the sh.o.r.e-haunt (littoral area) are the s.p.a.cious, bountiful, and relatively easygoing conditions of the open sea (pelagic area), which means the well-lighted surface waters quite away from land. Many small organisms have their maximum abundance at about fifty fathoms, so that the word "surface" is to be taken generously. The light becomes very dim at 250 fathoms, and the open sea, as a zoological haunt, stops with the light. It is hardly necessary to say that the pelagic plants are more abundant near the surface, and that below a certain depth the population consists almost exclusively of animals. Not a few of the animals sink and rise in the water periodically; there are some that come near the surface by day, and others that come near the surface by night. Of great interest is the habit of the extremely delicate Ctenoph.o.r.es or "sea-gooseberries," which the splash of a wave would tear into shreds. Whenever there is any hint of a storm they sink beyond its reach, and the ocean's surface must have remained flat as a mirror for many hours before they can be lured upwards from the calm of their deep retreat.

The Floating Sea-meadows.

To understand the vital economy of the open sea, we must recognise the incalculable abundance of minute unicellular plants, for they form the fundamental food-supply. Along with these must also be included numerous microscopic animals which have got possession of chlorophyll, or have entered into internal partnership with unicellular Alg- (symbiosis). These green or greenish plants and animals are the producers, using the energy of the sunlight to help them in building up carbon compounds out of air, water, and salts. The animals which feed on the producers, or on other animals, are the consumers. Between the two come those open-sea bacteria that convert nitrogenous material, e.g. from dead plants or animals that other bacteria have rotted, into forms, e.g. nitrates, which plants can re-utilise. The importance of these middlemen is great in keeping "the circulation of matter" agoing.

[Ill.u.s.tration: 1. SEA-HORSE IN SARGa.s.sO WEED. In its frond-like tags of skin and in its colouring this kind of sea-horse is well concealed among the floating seaweed of the so-called Sarga.s.so Sea.

2. THE LARGE MARINE LAMPREYS (PETROMYZON MARINUS), WHICH MAY BE AS LONG AS ONE'S ARM, Sp.a.w.n IN FRESH WATER. Stones and pebbles, gripped in the suctorial mouth, are removed from a selected spot and piled around the circ.u.mference, so that the eggs, which are laid within the circle, are not easily washed away.