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.