The first feeble ancestors of the shark and the sturgeon appear at a time when the crustaceans were the most powerful animals in the world, and the huge, lobster-like Pterygotus was the monarch of the
seas. The plated-scaled fish which existed at the same time were clumsy creatures, for their skeletons were probably feeble, and their armor-like shields were heavy. So, as history went on, they gradually gave way, becoming smaller and rarer, while the more active little shark-like animals gradually grew strong and powerful, and from them are descended the giant sharks of to-day.
The powerful gristly-boned fishes are much excelled in agility by the herring, the salmon, and their other bony companions, which move with much less effort in the water, and so have naturally 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 enameled-scaled fish, and struck out a path of their own, to make the most of the watery world.
If we wanted to pick out the strangest and strongest proof of how the shape of fish is altered to suit their wants, we need seek no further