Stories of the Universe: Animal Life.
by B. Lindsay.
PREFACE
Of the diagrams which ill.u.s.trate this little volume, the majority were prepared by Miss E. C. Abbott (formerly Bathurst Scholar at Newnham College, Cambridge): the sketches were made from specimens in the South Kensington Museum of Natural History, which has kindly granted permission for their use. In addition to these, there are several figures that are taken from specimens in my possession, photographed by the publishers; two or three cuts are diagrammatic; and I owe to the kindness of Mr. J. Craggs, formerly president of the Northumberland Microscopical a.s.sociation, the drawings of Polycystina and of the scales of the Sole.
B. L.
CHAPTER I
THE STORY OF ANIMAL LIFE
If the microscope had never been invented, the Story of Animal Life, as it is related by modern science, could never have been told. It is to the microscope that we owe our knowledge of innumerable little animals that are too small to be seen by the una.s.sisted eye; and it is to the microscope that we owe the most important part of our knowledge about the bodies of larger animals, about the way in which they are built up, and the uses of their different parts. The earlier opticians who toiled, one after another, to bring the microscope to perfection, never dreamed, in their most ambitious moments, of the value of the gift that their labour was to confer upon mankind. For the microscope alone has made it possible for men of science to study the world of living things. This is the value of honest and thorough work in almost every department of intellectual labour; that it builds a firm and sure though perhaps hidden foundation for the loftier and more perfect work of after days.
The microscope has shown us the intimate structure of every organ of the animal body; and thus, in most cases, the uses of the organ, and the steps by which it performs its tasks, have been made clear. The microscope has also shown the true nature of the s.e.xual functions, and all the steps of the processes of growth in young animals. None of these things could ever have been rightly understood without the microscope, for all their most important details are invisible to the naked eye.
To the microscope, too, we owe our knowledge of the essential kinship between plants and animals; to it, also, our understanding of the oneness, the "solidarity," as the French would say, of the animal kingdom, for it is in the structure of microscopic parts that resemblances are revealed under the most strikingly different circ.u.mstances of outward form.
Let us inquire a little into the history of the animals that can only be seen by the aid of the microscope. Most of them live in water, especially dirty water, containing decaying remains of plants or animals. The naturalists who first discovered them studied them in "infusions" of hay, and so on, and hence these little creatures were named Infusoria--a name that has since been somewhat restricted in its application. By an "infusion" is meant that water is poured on some substance and allowed to stand; the more ancient and evil-smelling the infusion becomes, the more of these little animals do you find living in it. Nature provides dirty water ready made, in ditches and in ponds, and these are full of microscopic animals. And not only do they appear in dirty water, but kindred kinds appear in clean water also, and many in the waters of the sea.
It will easily be understood that when the existence of microscopic animals was discovered, zoologists had greatly to modify their ideas of the animal world. Still more was this the case afterwards, when it was found that all animals were built up of minute parts much resembling these microscopic animals in their main features. To these unit parts, of which all animal bodies are composed, the term "cell" is applied.
The name of cell is not very descriptive of these units in the animal body, but correctly describes the unit of plant structure. In certain important essential particulars both, however, are alike. Nowadays we are not content to describe the grouping and external features of cells; their minute structure also is made a subject of research and inquiry, and affords a field for most of the fashionable speculations of our own day.
How great has been the progress made by the science of zoology since the eighteenth century may be estimated from the following quotation:--
"I remember," says the late George J. Romanes (in his book called "The Scientific Evidences of Organic Evolution"), "once reading a very comical disquisition in one of Buffon"s works on the question as to whether or not a crocodile was to be cla.s.sified as an insect; and the instructive feature in the disquisition was this, that although a crocodile differs from an insect as regards every conceivable particular of its internal anatomy, no allusion at all is made to this fact, while the whole discussion is made to turn on the hardness of the external casing of a crocodile resembling the hardness of the external casing of a beetle; and when at last Buffon decides that, on the whole, a crocodile had better not be cla.s.sified as an insect, the only reason given is, that as a crocodile is so very large an animal it would make "altogether too terrible an insect.""
How different is the state of knowledge now, when every part of a crocodile or a c.o.c.kroach is described in print in the minutest detail, and set before even the beginner in zoology as a necessary lesson.
But in spite of the labour necessary to master such detailed lessons, the study of the animal world is far from prosaic. The Story of Animal Life, indeed, bids fair to be the only element of romance left in the modern world for those who stay at home in their own land. The traveller of days of yore, when he ventured into the woods and fields, or upon the water, expected to meet with all sorts of strange things--fairies and elves and ugly gnomes; giants, ogres, and dragons; mermaids and water-witches. With the spread of education all these things have vanished now; it is quite certain that no Board-School-boy has ever met any of them: and one"s walks abroad would be in these days as prosaic as they are safe, but for the world of animal life. If you have eyes for this, every field has its inhabitants, and every hedge its marvels.
Instead of a fairy, you may be well contented to meet a dragon-fly with shining wings; instead of an ogre you will find the fierce spider, which not only makes away with every harmless fly that blunders into her net, but in many cases destroys her own kind also. Many a plant may be met with which has its own special caterpillar or other dependent insect, with ways of its own, which may amuse your idle hours. As for the change of a caterpillar or a tadpole into its adult form, it would be taken for a miracle if it were observed for the first time.
The reader may have noticed that there are some unfortunate people who have no eyes for these things; from childhood upwards they have been so absorbed in money-making or in reading books--the one case is as bad as the other--that they have never learnt to observe the facts of nature.
Some cannot even recognise the different kinds of plants that they see in the hedges, or in a country walk. Such natures are intellectually defective; they are much to be pitied, and require a special training to remedy their stupidity. I mention this, because the occurrence of this form of stupidity is one of the dangers resulting from town life and bookish education, which we have to guard against at the present time.
But for all healthy people accustomed to the outdoor world, the study of animal life has always possessed an interest. Its interest has, however, been increased a hundred fold by the progress of modern discovery, which has taught us to see in the animal kingdom one large family, working its way upwards from humble beginnings, to more perfect structure of body, and more complete intelligence of mind.
CHAPTER II
HOW ANIMALS ADAPT THEMSELVES TO CIRc.u.mSTANCES
We all know what it is to adapt ourselves to circ.u.mstances. Suppose two lads, fresh from school, go out into the world to earn their living; one becomes a navvy and one a clerk. In five years" time these two young men will probably be very different in appearance from one another.
The navvy will have developed his muscles; he will be broad-built, broad-chested, and strong. The clerk, on the other hand, will probably be comparatively weak and slim, his chest will not be so broad, his muscles will not be so well developed. The navvy, too, will probably be of a fresh complexion, while the clerk will be pale. All these differences are due to the fact that their bodies have adapted themselves to circ.u.mstances. Both men may be equally healthy, and equally long-lived. Let us take another example. Let us compare two other youths, of whom one becomes a cobbler and one an Alpine guide.
The latter, in five years" time will have become a perfect specimen of muscular humanity--active, agile, and hardy. The cobbler will be comparatively stiff in his limbs and unable to undertake any singular feat of muscular exertion, although he may be able to do a very hard day"s work at his own trade. The mountaineer, too, will probably differ in disposition from the cobbler. He will be daring, resourceful, and not afraid of danger under circ.u.mstances which would terrify the cobbler.
Now let us suppose that the sons and grandsons of the navvy are brought up to be navvies, and the sons and grandsons of the clerk are brought up to be clerks;--that the children and grandchildren of the Alpine guide follow his own calling, and the children and grandchildren of the cobbler do the same;--we shall probably have four families differing very much in type of physique from one another. Yet take one of the navvy"s st.u.r.dy grandchildren and bring him up as a clerk, and he will lose much of his st.u.r.diness. Let the mountaineer"s grandsons be brought up as cobblers, and by the time they are thirty they will not be remarkable for their muscular capabilities.
Just in a similar way the bodies of animals adapt themselves to circ.u.mstances. It is not always possible to trace the steps by which this has been done. But sometimes it is so; and we may find a whole series of varieties that are plainly due to adaptation. When we see an animal which is in some way especially fitted for its surroundings, we are therefore justified in concluding that it has become so by degrees.
The way in which animals adapt themselves to their surroundings in the matter of colour would afford material for several volumes each as large as this one. Those who have not travelled in foreign countries may perhaps find it difficult to realise that brilliant colouring and showy patterns can ever enable an animal to hide itself successfully. But an instance may be taken from an animal common on our own sh.o.r.es which will ill.u.s.trate how this principle works.
In the spring there may be found in large numbers upon our rocky coasts a little oval sh.e.l.l-fish, about one-third of an inch long, sticking to the fronds of the tangle and other broad-leaved seaweeds. The animal is of a very pale brown colour; its sh.e.l.l brownish and semi-transparent, with several stripes of brilliant turquoise blue down the back. These stripes are not continuous, but interrupted at intervals so as to give them a beady look. Taken in the hand and looked at closely, the sh.e.l.l, with its contrast of blue stripes on a brown ground, is extremely conspicuous; brown being, in fact, the contrast-colour which shows blue in its greatest brilliancy. Yet, when perched upon the tangle, the creature is almost invisible, and might easily be mistaken for a natural irregularity of the surface of the seaweed. While the brown is the colour of the seaweed itself, the brilliant blue is indeed the exact colour of the spring sky at that season, everywhere reflected from the sea-water and from the wet surface of the seaweed. By matching that brilliant colour the animal therefore is rendered invisible. This little creature is the young of the Semi-transparent Limpet, _Patella pellucida_. This, at least, was the old-fashioned name for it, though it has received others. Its young and its adult form are so different in the appearance of the sh.e.l.l, that they have been described under different names. English readers who search for it in the spring will learn by experience that bright colouring may help to make a creature invisible. But this is not all that is to be said about the protective colouring of this little sh.e.l.l-fish. There are many creatures whose young live at the surface of the sea, and afterwards migrate to deeper water as they attain adult age. In early life they are transparent, because thus they best escape notice in the clear water of the surface, especially when seen from below, by the many enemies on the watch to devour them. But in their later life they become opaque, because thus they best escape notice from enemies watching from above, as they crawl along the bottom of the sea. Now this is the case with the little Patella. For this also migrates to the bottom--in this instance a comparatively short journey--when it is ready for adult life. Both sh.e.l.l and animal, therefore, are at first nearly transparent, but in older life both become more opaque; the blue stripes, too, are almost or quite obliterated in the after-growth of the sh.e.l.l, slight traces of them alone remaining at its apex. This change of colour fits the animal for the new home in which it settles, for it moves down from the leaf of the tangle to its root, and there finds a snug shelter among the coral-shaped branches of which the root is composed. Not many reflections of the blue sky are likely to reach the recesses of the tangle-root, so the creature has no longer any need of its protective colouring of blue.
The adult sh.e.l.l, however, retains a certain degree of translucency, which matches very well with the colouring of the tangle-root; and thus presents a great contrast to the sh.e.l.l of the common Limpet, which is found on rocks. The rugged surface of the latter is usually more or less irregularly speckled in harmony with the surfaces on which it lives, though this sh.e.l.l also presents when young occasional touches of blue, which suggests a family likeness in colour tastes on the part of the two kinds of Limpet. The blue in this case, however, is of the dullest and dingiest shade. The _Patella pellucida_ is common on the more rocky portions of our coasts; in spring the young may be seen in thousands on the seaweeds of the Isle of Man; here its habits were first observed and described in detail by the Manx naturalist Forbes, who noticed its peculiar way of finding a hiding place among the roots of the tangle.
The same sh.e.l.l-fish, in contrast with the commoner Limpet of the rocks, affords another instance of the way in which sh.e.l.ls adapt their forms to their surroundings. In each case the sh.e.l.l is a plain conical cap, and the animal within keeps the sh.e.l.l firmly attached to the base on which it rests. The Limpet can move about at a very creditable snail"s pace when it wishes to do so, and at low-water mark, when the tide is beginning to rise, you may easily find them moving about and off their guard; but during many hours of the day, when the tide is out, the main object of the Limpet is to keep its sh.e.l.l as firmly fixed to the rock as possible. It will at once be seen that if the margin of the sh.e.l.l were smooth like that of a tea-cup, and the surface of the rock to which it clung very irregular, many c.h.i.n.ks would be left between the margin of the sh.e.l.l and the surface of the rock through which unwelcome visitors might find entrance. The loss of moisture through the crevices, too, would be a serious thing to the animal during the hours when the sh.e.l.l is uncovered by the tide and exposed to the rays of a hot sun. On the other hand, if the margin of the sh.e.l.l were irregular, and the surface on which it rested smooth, unprotected crevices would in the same way be left. So the Limpets adapt the shape of their sh.e.l.l to their surroundings; the _Patella pellucida_, which lives on the smooth branches of the tangle-root, has a sh.e.l.l with a smooth regular edge; while the _Patella vulgata_, which lives upon rocks, has a sh.e.l.l with an irregular, indented edge, whose irregularities fit into those of the rock on which it rests. (See Fig. 2.)
[Ill.u.s.tration: FIG. 2.--Sh.e.l.ls mentioned in Chap. II. 1, Common Limpet, old and young; 2, Semi-transparent Limpet, old and young (the remains of the young sh.e.l.l may be seen crowning the adult sh.e.l.l); 3, Common Yellow Periwinkle; 4, Common Edible Periwinkle; and 5, High-tide-mark Periwinkle, both with a sharp spire, for comparison. One specimen of the latter stands among group 3.]
Probably every reader will be able to appreciate the above instances of creatures adapted to their surroundings. For there are few people who are not familiar with the common Limpet of the sh.o.r.e between tide-marks, and with the great seaweed called Tangle, which has its habitat a little lower down, and forms great sea-meadows, whose upper limits alone are ever laid bare by the tide. The _Patella pellucida_, too, is fairly common, and the dead sh.e.l.l may be found on most rocky parts of our coast all the year round. As for the blue-striped young sh.e.l.l, floating on the blades of the tangle, those who have leisure to visit the seaside during the months of spring and early summer, may have seen it as I have described it; and the mention of it will recall pleasant memories of clear spring skies, and fresh sea-winds, and fields of heavy tangle swaying gently on the swell that comes in from the open sea. It is interesting to know something of the habits of the creatures whose forms we study, and we have already spoken of the snug little hiding-place that the Semi-transparent Limpet finds for itself in the tangle-root.
It is of interest to remember that the Common Limpet, too, is a home-loving creature, which knows and prefers the spot of rock on which it habitually rests; and can find its way back to it, aided by its two eyes and two smelling patches. This has been proved by Professor Lloyd Morgan, who has recorded the result of his observations, made on the coast of Dorsetshire. It is not easy to detach a Limpet from the rock without injuring or exhausting it, but these specimens were caught when moving of their own accord, and were therefore uninjured and brisk. They were removed to short distances, and the following table shows the result of the experiment, clearly proving that the Limpet prefers home, but regards a distance of two feet as a very long journey.
----------+-------------+----------+---------+--------NumberNumberDistance inReturnedIn FourLater.
Removed.Inches.in TwoTides.Tides.----------+-------------+----------+---------+-------- 2562100 21121350 21181062 3624113 ----------+-------------+----------+---------+--------
Similar observations were made at an earlier date, by Mr. George Roberts, at Lyme Regis.
Let us now take an instance of adaptation in form. And this time we will take a sh.e.l.l so common that everybody will know it.
Everyone who has spent a little time in naturalising on the sh.o.r.e, has noticed how often you may find univalve sh.e.l.ls, such as those of the whelk and periwinkle, with the top of the sh.e.l.l knocked off. This is nearly always the case with the dead sh.e.l.ls that you find strewn along the tide-line; and after a storm, on a rocky coast, you may find sh.e.l.ls that still contain the living tenant, in the same sad condition. And you may also meet not infrequently with sh.e.l.ls, dead or living, that bear evidence of the owners" efforts to repair them after an accident to the spire. A piece has been broken, and you find it cemented on again by a patch of sh.e.l.l, serviceable no doubt to the owner, but crooked and unsightly in appearance. Now there is a very common sh.e.l.l, the little yellow periwinkle, which has practically done away with its spire, the coils of the sh.e.l.l being so curved that the earlier part of the spire does not project beyond the later-formed coils, and the whole sh.e.l.l has a rounded outline. This little creature lives on the long seaweeds which grow at low-water mark or near it; and when the sea is rough it is obviously liable to be dashed from its foothold on the seaweed and flung violently down, as the huge seaweeds sway about in the shallow waves. We may easily satisfy ourselves that this is an accident that frequently happens, by examining the sh.o.r.e when the tide is going out, on some stormy spring or autumn day. Numbers of the yellow periwinkles are then to be found crawling on the sand, and striving to regain their place in the seaweedy rocks as soon as possible. On a calm day you will rarely see one crawling on sand above low-water mark, for it is a place they do not choose by preference; those that are to be found there on the stormy day have lost their foothold, and have been washed about by the tide.
Had they, like some other kinds of periwinkle, a sharp spire, how many would be the casualties under these circ.u.mstances! But as it is, you do not see a single specimen with a broken top: the rounded spire is an adaptation to circ.u.mstances, required for the protection of the tenant of the sh.e.l.l. (See Fig. 2.)
It may be added that the yellow Periwinkle is not only protected from mechanical sources of danger by its form, but is also in some degree protected from living enemies by its colour. This, at first sight, seems exceedingly conspicuous. We must remember, however, that the animal often lives in that part of the sh.o.r.e where the Bladder Seaweeds, or Fuci, are extremely abundant. The flowering ends of these are of a yellow colour, fairly bright. When seen from below, with the sunlight streaming through them, they no doubt appear much brighter than when seen, as we see them, from above, with the sunlight falling on them.
Now protection from foes below is what the yellow periwinkle needs most: for fishes are quite ready to swallow it whole, and are not in any way deterred by the thickness of the sh.e.l.l, which is (by-the-way) in a measure a protection against birds when the tide is out; fishes habitually swallow sh.e.l.l-fish whole, although the inmate only is digested. The bright yellow, then, that seems to us so conspicuous, is probably a good means of hiding for the periwinkle when under water. Its common variations in colour, too, are probably protective in their use: some are a dull purplish brown, some drab. These are good colours in which to lie hidden, respectively, under darker tracts of seaweed, or upon the rock itself. This little sh.e.l.l is so abundant on rocky coasts that on some beaches the dead sh.e.l.ls are as numerous as pebbles. No wonder, with all these adaptations for protection!
Another instance of adaptation to circ.u.mstances is described in the sea-urchin shown on p. 125. This is one among many instances where animals that live on sand or mud acquire a flattened shape, so that their weight is distributed, and the danger lessened, of their sinking in a quick-sand. The flat-fish, such as soles and flounders, are a familiar example; and the same principle is ill.u.s.trated by the flattened forms of many of the bivalve sh.e.l.l-fish, whose flat sh.e.l.l, when closed, can lie safely on the loosest sand. Equally is their form adapted for their circ.u.mstances, when, in their slow way, they begin to move. For the flat valves of the sh.e.l.l are placed to the right and left of the animal"s body. So that when it stirs, or floats quietly in the current of the tide, the sh.e.l.ls present their sharp edges to the resistance of the water, thus enabling the creature to move like a ship through the sea, or like a knife-blade through bread, with the least possible friction: and specially is this provision for the lessening of friction important, when we consider that many of these bivalve sh.e.l.l-fish have to move, not only through water, but also through sand and mud.
It may be a.s.sumed that every reader is familiar with the common forms of the bivalve sh.e.l.l-fish. The frontispiece shows one of them, considerably flattened in shape.
So far, however, we have not explained _how_ animals adapt themselves to circ.u.mstances; we have only pointed out the fact that they do so.
Take the case of our little Limpet. It cannot say: "I will paint myself with blue and brown, so as to be mistaken for a bit of seaweed reflecting the blue sky"; nor can the periwinkle say: "I will paint myself with yellow, so as to pa.s.s unnoticed among the yellow ends of the _Fucus_; and I will build my spire low, so that it will not be broken."
The bivalve sh.e.l.l-fish and the Sand-Cake sea-urchins do not say to one another, "Let us alter our sh.e.l.ls, and build them a little flatter, so that we shall not sink in too deep when we lie upon the ooze and sand of the sea."
How then do these adaptations take place? Darwin has explained this for us. Individuals often have some little peculiarity, in which they differ from the average of their kind. The establishment of such little marks of individuality is spoken of as Variation. If among these individual peculiarities there is one which is in any way disadvantageous, _e.g._ one which tends to make the creature conspicuous in the sight of its foes, the owner will be quickly eaten, and of that peculiarity there will be an end. If, on the contrary, the peculiarity gives the owner some advantage over its fellows, that individual will survive, and probably transmit its peculiarity to some of its descendants.
We have seen, for instance, that it is of advantage to our little periwinkle to be yellow, when it lives in certain situations; and that it sometimes presents other colours, likely to be favourable in other cases. If we gather together a large number of specimens, we shall find a surprising range of variation in colour. Some present a tint of bright orange, nearly red; some are a dull brown; the dark purple shade and the drab have been already referred to. The very young sh.e.l.l usually presents an unmistakable shade of pink; and we may find innumerable half-grown specimens in which we may trace the gradual establishment of the advantageous yellow colour, from an original shade of unmistakable pink, presented by the earlier whorls. Kindred varieties of the sh.e.l.l, too, may be found with stripes or speckles. Since this very common sh.e.l.l may be found in abundance on any rocky sh.o.r.e in the British Isles, the reader may easily study its colour-variations, both in the dead and the living sh.e.l.l. Study also the ground on which the creature lives, with its sharp colour-contrasts of rock and seaweed patches, and it will be easy to understand why the colours are thus varied, with a preponderance, on the whole, of the yellow shades. It is all a question of the survival of the fittest--the unfit being represented by colours too easily seen, and therefore quickly snapped up. As for the spire, it has already been shown how that is adapted to circ.u.mstances. It is worthy of remark that in the kindred Edible Periwinkle, _Littorina littorea_, which has a sharp spire, elderly specimens may be seen with the end of the spire damaged.
Turn again for a moment to our first instance--the adaptation of men to a sedentary or an outdoor occupation. Here we dwelt upon the change produced by their mode of life; we left out of sight the "survival of the fittest." Yet here it is equally surely at work. How often does the young mountaineer, less agile than his fellows, come by a violent death?
Only those who are equal to the necessities of the life survive--many are lost. How often does the clerk, tied to his desk, fail in health and die? How often, hating a sedentary life for which he is unfitted, does he throw his energies into athletics, lose interest in his office work, and get dismissed? Here again comes in "the survival of the fittest"--for a desk: alas! perhaps the only means of livelihood.