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We all know what it is to adapt ourselves to circumstances. 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 circumstances. 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 circumstances 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 sturdy grandchildren and bring him up as a clerk, and he will lose much of his sturdiness. 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 circumstances. 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 shores which will illustrate how this principle works.

In the spring there may be found in large numbers upon our rocky coasts a little oval shell-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 shell 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 shell, 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 shell, 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 shell-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 shell 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 shell, 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 shell, 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 shell 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 shell 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 shell-fish, in contrast with the commoner Limpet of the rocks, affords another instance of the way in which shells adapt their forms to their surroundings. In each case the shell is a plain conical cap, and the animal within keeps the shell 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 shell as firmly fixed to the rock as possible. It will at once be seen that if the margin of the shell were smooth like that of a tea-cup, and the surface of the rock to which it clung very irregular, many chinks would be left between the margin of the shell 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 shell is uncovered by the tide and exposed to the rays of a hot sun. On the other hand, if the margin of the shell 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 shell to their surroundings; the Patella pellucida, which lives on the smooth branches of the tangle-root, has a shell with a smooth regular edge; while the Patella vulgata, which lives upon rocks, has a shell with an irregular, indented edge, whose irregularities fit into those of the rock on which it rests. (See Fig. 2.)

Fig. 2.—Shells mentioned in Chap. II. 1, Common Limpet, old and young; 2, Semi-transparent Limpet, old and young (the remains of the young shell may be seen crowning the adult shell); 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 shore 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 shell may be found on most rocky parts of our coast all the year round. As for the blue-striped young shell, 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.

Number Removed.	Distance in Inches.	Number Returned in Two Tides.	In Four Tides.	Later.
25	6	21	0	0
21	12	13	5	0
21	18	10	6	2
36	24	1	1	3

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 shell so common that everybody will know it.

Everyone who has spent a little time in naturalising on the shore, has noticed how often you may find univalve shells, such as those of the whelk and periwinkle, with the top of the shell knocked off. This is nearly always the case with the dead shells that you find strewn along the tide-line; and after a storm, on a rocky coast, you may find shells that still contain the living tenant, in the same sad condition. And you may also meet not infrequently with shells, 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 shell, serviceable no doubt to the owner, but crooked and unsightly in appearance. Now there is a very common shell, the little yellow periwinkle, which has practically done away with its spire, the coils of the shell being so curved that the earlier part of the spire does not project beyond the later-formed coils, and the whole shell 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 shore 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 circumstances! But as it is, you do not see a single specimen with a broken top: the rounded spire is an adaptation to circumstances, required for the protection of the tenant of the shell. (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 shore 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 shell, which is (by-the-way) in a measure a protection against birds when the tide is out; fishes habitually swallow shell-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 shell is so abundant on rocky coasts that on some beaches the dead shells are as numerous as pebbles. No wonder, with all these adaptations for protection!

Another instance of adaptation to circumstances 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 illustrated by the flattened forms of many of the bivalve shell-fish, whose flat shell, when closed, can lie safely on the loosest sand. Equally is their form adapted for their circumstances, when, in their slow way, they begin to move. For the flat valves of the shell 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 shells 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 shell-fish have to move, not only through water, but also through sand and mud.

It may be assumed that every reader is familiar with the common forms of the bivalve shell-fish. The frontispiece shows one of them, considerably flattened in shape.

So far, however, we have not explained how animals adapt themselves to circumstances; 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 pass unnoticed among the yellow ends of the Fucus; and I will build my spire low, so that it will not be broken." The bivalve shell-fish and the Sand-Cake sea-urchins do not say to one another, "Let us alter our shells, 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 shell 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 shell, too, may be found with stripes or speckles. Since this very common shell may be found in abundance on any rocky shore in the British Isles, the reader may easily study its colour-variations, both in the dead and the living shell. 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 circumstances. 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.

But why do variations occur? This is the question first asked by a child, when you try to explain the working of "natural selection." It is also the last question asked by scientists, who are still industriously engaged upon studying the problem.

In the above instances from human life, we have considered the occurrence of changes brought about in the organism by the circumstances of life; or as scientists say, by the "environment." Scientific men are busily hunting for instances of variation of this sort. Take for example, an animal which lives sometimes in salt water, sometimes in water that is only brackish; there are cases in which small differences can be noticed, according to the difference in the habitat. Notice the marine shell-fish, for instance, near the estuary of a river: they are often less robust specimens than are found at a point free from the influence of fresh water.

Not until the effect of known causes on the rise of variations has been studied much more fully than at present, will it be possible to judge regarding the nature of those variations which appear to be spontaneous; for which, at present, no predisposing cause can be assigned.

A very large number of variations, however, fall into the class of "Atavistic" variations; that is to say, those which show a return to an ancestral type. These are variations which are very rarely welcome. If, for instance, a boy has a pair of handsome black rabbits, he is not much pleased to find among their progeny, every now and then, one of the colour of the original wild Bunny. The probability, in this case, is that the atavistic variety will find its way into a pie, instead of being kept as a pet. Equally unsatisfactory to the owner, is the incorrigibly savage and intractable dog or horse—a reversion to the mental type of an ancestor which knew not the authority of a master.

Atavistic variation often occurs when members of two well-marked varieties are mated; so that in some of the offspring produced, each parent seems to cancel out the more extreme characteristics of the other, leaving only the characteristics of the more generalized ancestral type, from which both parents have alike been derived.

When the ancestral type is in some way inferior to the modern one, variation which consists in reverting to the former is often referred to as Degeneracy. There is reason to believe that discomfort and hardship of existence tend to produce variation of this kind—a fact of supreme importance, when the problem of Degeneracy is considered in connection with human life. When creatures begin to degenerate, it is, in fact, as if the species were saying to itself, "I have gone astray; let me retrace my steps along the road by which I came, and maybe I shall find comfort and safety; step by step I will try to go back to my ancestral form."

Very rapid variation of any sort is indeed often a sign that the struggle for existence is too hard for the type in question. The palæontologist can tell us of types that present numerous variations before becoming extinct; while others, comfortably holding their own in the struggle for existence, remain practically unchanged during age after age of the geological record, and survive even up to the present day. We may borrow from commercial life a homely illustration that will explain this aspect of variation. When competition in trade is keen, the seller must have novelties; he will try all sorts, and find some good, some bad, some indifferent. If he now revives an out-of-date pattern of goods, for the sole sake of change, this is Degeneracy. But where, on the contrary, competition is dull, the same firm will turn out the same goods for a long period of time. There is an optimum in trade competition: a reasonable competition results in the production of sensible novelties, and consequent progress; but competition over-keen results in the production of rubbish, leading to eventual failure. So in the world of animal life; a certain degree of struggle for existence results in variation, establishment of new varieties, progress. A greater degree results in too rapid variation, new varieties that speedily perish, and finally, the extinction of the type.

We have spoken of "varieties." Each of the domestic animals presents varieties, which are the cumulative result of the breeder's artificial selection of natural variations. Thus the Pug and the Collie for instance, are varieties of the Dog; the Bantam and the Dorking of the Fowl. Among wild animals, varieties are similarly produced by natural selection, resulting from the "survival of the fittest." By degrees, intermediate forms are lost; and new species are established by the greater and greater divergence of varieties originally derived from one ancestral type.

Table Showing the Position in Classification of the Animals Named in the Foregoing Chapter

Phylum	MOLLUSCA, or Shell-fish.
Class	GASTEROPODA, or Snail-like Shell-fish.
Sub-Class	Anisopleura, or Unequal-sided Gasteropods.
Branch	Streptoneura, or Unequal-sided Gasteropods with nerves twisted into the shape of a figure of 8.
Order	Zygobranchiata, or Streptoneura with a pair of gills.	Azygobranchiata, or Streptoneura a pair of gills.
Genus	Patella, the Limpet, with gills obliterated, a pair of gills. and only indirectly represented; breathing is performed by folds of the mantle.	Littorina, the Periwinkle, or Shore Shell.
Species	Vulgata, the Common Limpet.	Littoralis, the (Yellow) Periwinkle that lives above low-tide-mark.