THE BONY FISH IN THE EARLY DAYS
CHAPTER III.
THE BONY FISH, AND HOW THEY HAVE SPREAD OVER SEA AND LAKE AND RIVER
When the palmy days of the enamel-scaled fish had passed away, and the sharks and rays had taken up their various quarters in different parts of the sea, there still remained vast tracts and many snug nooks and bays admirably fitted for fish-life. But these were not empty, for long before this time another order of fish—light, strong, and active,—had been pressing forward to take possession of every vacant space.
If we could dive under the water and watch the fishes at home we should see at once how much more agile and easy the bony fish are in their movements than their gristly companions. Look at a shoal of silvery herrings as they swim and leap and gambol, or a fine salmon sailing up the river or springing over a waterfall, or a tiny stickleback darting across the stream, and compare their graceful motion with the ponderous though powerful movements of an unwieldy shark. Any one who has done this will feel at once that though the sharks have still kept their power as tyrants of the sea, because they are so strong and big, yet these light skirmishers are much more at their ease, and move with much less effort in the water, so that it is natural they should have made their way into all parts of the rivers and seas. But where have they come from? We know very little of their early history, but what little we do know leads us to think that long ago they branched off from the enamel-scaled fish, and struck out a path of their own to make the most of the watery world.
Turn back for a moment to our little minnow, and recall his tender backbone made of joints hollowed out before and behind, with cushions of gristle between; those cushions, when the minnow was growing out of the minnow egg, were one long gristly cord, like the cord of the sturgeon, and it was only as the minnow grew that the bony joints hardened round it and separated it. Moreover, that huge bony pike which we find now wandering in the American lakes has bony joints hollowed out like the minnow’s, although by his enamel-scales and uneven tail we know him to be one of the ancient fishes. Some time or other, then, the sturgeon, the bony pike, and the modern minnow, must have had a common ancestor, though we should have to reach him through millions of generations. In the same way, too, we find the red-folded gills covered by a scaly lid, both in the sturgeon and the minnow, though in other ways they are not exactly alike; while even the V-shaped tail of the modern fish is not so different from the ancient shape as it seems, for the end of the backbone runs up into the top branch of the fork as it does in the uneven tails of the olden fish. Lastly, the delicate rounded scales on our minnow’s body are not entirely the property of bony fishes, for we find such scales on the mud-fishes, the Amia and Ceratodus (see [p. 33]); while the little modern stickleback, on the other hand, has bony plates, reminding us of those of olden times. We see, then, that the bony fish still carry upon them many signs of their origin from the older fish, and when once the coast was left clear, and they got a fair start, we can easily imagine that the fish of this younger race which was still in its childhood, and easily moulded to suit different kinds of life, would press forward in every direction and make the most of every chance.
And so we find that little by little, from the time of those chalk seas till now, the remains of enamel-scaled fish grow rarer and rarer in the hardened mud, and the bones and scales of modern fish take their place, till this bony race has spread so far and wide that in our own day, if we were to start from the head of a river and swim down into the open sea of the Atlantic or Pacific, we should meet on our way bony fish of all shapes and sizes and habits of life. River-fish and lake-fish and sea-fish; shore-fish, surface-swimming fish, and fish of the deep sea; flat-fish like the sole, half hidden in the sand, and long rounded fish like the eel, threading their way through holes and passages all over the world; flying fish with long arm-fins, and clinging fish whose fins form a sucking disk; nay, even so strange a thing as an angling-fish, whose back fin is turned into a fishing-rod to attract his prey.
All these, during the long ages since they first started in life, have been learning to make use of some area in the wide expanse of water spread over our globe, and it remains for us now to see how they have succeeded. Where shall we make our start? If we begin at home in the rivers we should have to work, as it were, backwards, for the sea is the chief home of fishes, and the rivers only the refuge of a few stray kinds. The sea-shore would be, perhaps, our truest starting-point, but then we should have to travel two different ways. Will it not be best to dive down first into the silent depths of the ocean, and learn what little is known of those which have taken refuge there? Thence we can rise up to the open sea, from there swim in to the shore, and then up the rivers and back to our own land-home.
It makes but little difference where we take our plunge into the deep sea, for changes of climate are scarcely or not at all known there, and the fish seem to wander over every part. Wherever it may be, then,—let us say in the seas of the Tropics, which have given us most of our specimens—let us dive down, down, till we reach about 1800 feet (300 fathoms).
... “For who can know
What creatures swim in secret depths below,