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.