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. But while we may dispute about the highest fish, there is no doubt about the lowest one. This is the lancelet. It is of the size and shape of a toothpick, translucent, scaleless, and almost finless, burying itself in the sand on warm coasts, in almost every region. The lancelet has no real bone in it, just a line of soft tissue blocking out the space where the backbone ought to be. It has no skull, nor brain, nor eyes, nor jaws, nor heart, nor anything in particular--just transparent muscle, spinal cord, artery gills, stomach and ovaries, with a fringe of feelers about the slit we call the mouth. And even these organs are rather blocked out than developed, yet it is easy to see that the creature is a vertebrate in intention and therefore essentially a fish--a fish and a vertebrate reduced to their lowest terms. You can go fishing almost anywhere, but whether it is good to do it or not depends on your reasons for doing it. There are about three good reasons for going a-fishing, one indifferent one, and one that is wholly bad. One good reason is that you may learn to know fish. Isaac Walton tells us that "it is good luck to any man to be on the good side of the man that knows fish." This is true, but you cannot learn to know fish unless you go forth to find them. There are about 15,000 kinds of fish in the world; 4,000 of them in North America, north of Panama. Now no man knows them all, not even on one continent, though some have written books upon them.