Some crawled into dead shells. Some stretched their slim, ribbon-like bodies out in the hanging seaweed. Some fled into caves, whither no one followed them, and where they lay hid for a whole geological age, until, seeing nothing, they had all gone blind. And some went down into the depths of the sea — two miles, three miles, five miles — I have helped haul them up to the light — and these went blind like the others, for the depths of the sea are black as ink and cold as ice. And even there they are not safe, for other fishes go down there to eat them. And some carry lanterns, large, shining spots on their heads or bodies, sometimes like the headlight of an engine. And with these flashing lanterns, these burglars of the deep hunt their prey. And these are hunted by others fish-hungry, too, who lurk in the dark and swallow them, lanterns, headlight and all!

And so, with all this eating and chasing and fighting and fleeing and hiding and lurking, it comes about that wherever there is decent water on land or sea there are fishes to match it. And every part of every fish is made expressly for the life the fish has to lead. If any kind failed to meet requirements, other fishes would devour and destroy it. So only the fit can survive and these people the water after their kind.

All kinds of fishes are good to eat except a few which are too tough, a few which are bitter, and a few that feed on poisonous things about the coral reefs and so become poisonous themselves. Some are insipid, some full of small bones and some are too lean or too small to tempt anybody, unless it be another fish. But this is their business, not ours, and they have flesh enough for the things they have to do.

The biggest fish is the great basking shark, which grows to be thirty-five feet long, and lies on the surface of the sea, like a huge saw-log, filling its great mouth with the little things that float along beside it.

The smallest of all fishes lives in the everglades of Florida and the streams that run out of them. You can find them in the little brook that runs through Jacksonville. I have netted them there with a spread umbrella, which will serve when you cannot get a better dip-net.

They are prettily barred with jet black on a greenish ground, and they belong to that group of top minnows to which Agassiz gave the name of heterandria. It is hard to say what is the highest fish — what is the one which has undergone the greatest modification of structure. Perhaps this place should be assigned to the sole, with its two eyes both on one side of the head, peering through the same socket, while the socket on the other side has no eye at all. Or perhaps we may place as highest some specialized form as the angler or the sargassum fish, which has the paired fins greatly developed almost like arms and legs, and which has a dorsal spine modified into a fishing rod, which has a bait at the end, hanging over the capacious mouth.

Agassiz put the sharks higher than all these bony fishes because, while lower in most respects, the sharks have greater brain and greater power of muscle. Others again might give the highest place to the lung fishes, fishes of the tropical swamps, with lungs as well as gills, and which can breathe air after a fashion when the water is all gone. These are not high in themselves, but they are nearest the higher animals, especially interesting to us because from such creatures in the past all the frogs and salamanders, and through these all the beasts that bite, the birds that fly and the reptiles that crawl are descended. These are near the primitive fish stock, the ancestors of true fishes on the one hand and of the land vertebrates on the other. As such, they partake of the nature of both. More correctly, their descendants have divided their characters. Their land — progeny lost the gills, scales and fins of the lung fishes, while their water descendants have lost their lungs, or rather the use of them, for the lung of the fish is generally a closed sac, called the air bladder. Sometimes it is only partly closed, and sometimes it is lost altogether.