The progress of life from its beginning until now has been upward, and in the main the latest forms of life are the highest forms; but this is a rule which has many exceptions. We have seen many of the lowest and earliest forms of life continue living and reproducing themselves in the present age, and also we have seen the many types of life remain which are not now in their highest development, but are degenerated examples of higher forms that existed long ago. The noblest type of life is found in vertebrate animals; but the vertebrates have many families, and it is only when we finally reach the chief class of vertebrates that we find the highest type of life. The first vertebrates that appeared were fish. In their distinguishing characteristic, the backbone composed of many vertebræ, they were a prophecy of all the wonderful orders of vertebrate animals. The fish were of several species, and had not sprung from any preceding type of animals. In their vertebrate construction they were something entirely new in animal life. The first fish of which we have any knowledge were not poor little weaklings, hardly capable of maintaining existence; on the contrary, they came bravely into a world swarming with great crustaceans and polyps and long-armed cuttle-fish, another fish-named creature which is not at all a fish. Among these huge and ravenous animals the new specimen held its own, and asserted its higher condition. When most of the great forms of life which had witnessed its arrival had perished with the closing of the age, the fish continued into the next era, and there thrived exceedingly, and had their turn at being world-masters. With each advancing age of the world the fish-class has gone on increasing and reaching new forms of beauty and excellence. The crustaceans, polyps, crinoids, and nautili, which preceded the fish, had their bony structure upon the outside, and the soft parts of the body securely placed within this covering. The fish came with its bones inside its body, thus depending for safety upon its agility and its teeth. The first fishes had thick, tough skins, without scales. Their skeletons were rather of cartilage, or gristle, than of bone, but their spines were long, and their teeth were particularly large and strong. They were about the size of a dog-fish,[39] and were probably even more hideous to behold. Among these fish, however, and perhaps the oldest of them all, were some that, while having no scales, wore great bony plates, like the side plates of a sturgeon.[40] From the coprolites found with these fossil fish we discover that they ate sea-snails, crinoids, which were unceremoniously bitten from their swaying slender stems, and lingulæ, which they tore off the submerged rocks, as cheerfully as a boy helps himself to cherries or blackberries.