We can never fully understand an animal until we know its life-history, but we can give some sort of an account, at least, of its development from birth to death. With some creatures, as with butterflies, moths, or birds, for example, this is easy enough, but with others this is by no means true. The life-history of the Sole is a case in point; only by the slow accumulation of facts has this been put together. But the result is most interesting, and without more ado we now proceed to relate it.

The cradle of the young sole, like that of its relatives, the plaice, turbot, and flounder, takes the form of a crystal globe of a jelly-like material, in the centre of which lies a smaller globe containing the germ which will grow into the young fish, a little store of food material, and a small quantity of oil, which seems to keep the whole afloat at the surface of the sea. This is the egg. It differs from the eggs of its relatives, in that the oil which it contains is distributed in the form of tiny drops, instead of being collected in one big drop, as in the turbot's eggs, for instance. The careful mother lays these eggs far out at sea and leaves them; if they were deposited near the land they would drift ashore and be destroyed. And in the illustration (fig. 1, egg) you will see what this water-baby looks like just before he quits his cradle.

In less that a month the little sole has grown enough to enter the world, but he is strangely helpless; a tiny little creature, perfectly transparent, mouthless and finless, so that he must drift helplessly, whithersoever the currents carry him. Though mouthless, he is not hungry, for there remains within him a certain amount of the nourishing yolk, which was stored up for this purpose, in his crystal cradle. This little food reserve is the cause of the rounded swelling on the under surface of the young sole in the illustration (fig. 1, a and b). In this picture you should note, first of all, the curious shape of the head, which is, as yet, only roughly modelled. There is no mouth, and the eye, as yet, is colourless. Along the middle of the back there runs a high fin, transparent as glass, and this is continued round the tail and forwards to the swelling caused by the yolk-bag. Over the whole are scattered a few patches of colour, in the shape of spidery lines and blotches, as yet only just dense enough to attract attention.

At six days old, as you will see (fig, 1, c), he has grown darker, and has developed a mouth and a tiny pair of breast-fins; but beautiful he certainly is not, judged by human standards of beauty. It often happens, however, that the outward mark of ugliness is but the sign of hidden peculiarities of unusual interest. Up to this point this baby sole is very like any other fish-baby; but from now onwards it enters on a most remarkable career. At six days old he shows all the promise of a well-grown fish; that is to say, his body is round and tapering, he has an eye in each side of his head, and both sides of the body are alike in colour—in other words, he is symmetrical.

The beginning of the change (fig. 1, d) is indicated by a disposition of the growing fish to lie on one side—the left—and at the same time the left eye begins to change its position, moving from the side of the head towards the crown of it! In a short time this point is reached, and passed, and not until the left eye has approached its fellow of the right side fairly closely does its progress stop! By this time the habit of lying on one side has become fixed, and the body has taken the characteristic shape of the sole. Thus, then, what appear to be the upper and under surfaces of the sole, are really the right and left sides, and this can easily be proved by a careful examination of the body, which, if it be placed on edge will be found to have a back or dorsal fin, and a pair of breast fins—one on either side, as in ordinary 'round' fishes.

The difference in the colouration of these two sides is a matter to which we must now refer. As everybody knows, the upper side is dark-coloured, while the under side is white. Why is this? Why are not the colours reversed, or why are not both sides coloured? These questions open up a most fascinating study—the use and meaning of the colours of animals. And you will find, when you come to look into the matter, that there is a very close relation between the colour of an animal and the nature of its surroundings. In the case of the sole, the brown upper surface, from its resemblance to the mud and sand at the bottom of the sea, serves to conceal it from the sharp eyes of prowling fishes on the look-out for a meal. A broad expanse of white would at once betray it to the enemy. No colour is developed on the under surface, for it would be a waste of energy to produce colour for a surface that was kept constantly concealed from view.

Although, in our picture, all these fish can be seen quite plainly, in real life they are quite hard to find. The young, being well-nigh transparent as glass, are almost invisible as they float in the water; while later, when these wanderings cease, and they settle down to a quiet life, the dark colour forms an equally invisible covering.

W. P. Pycraft, F.Z.S., A.L.S.