In the study of fishes we notice that some are highly specialized so far as their structures are concerned ; the teeth of some become especially fitted for a peculiar kind of food, and as a result quite unfit for any other kind. Some, to be protected from their enemies, develop a heavy armor, which only retards their activity. Other fishes are more generalized; that is, are of medium size, omnivorous habits, are not hampered in their movements by a too heavy coat of mail, etc. When any change of conditions came to modify their habits of living the specialized were always the first to disappear. Being particularly fitted for one mode of life made them all the more unfitted for any other, and so when conditions changed they perished. All of our modern fishes except the few ganoids are more or less specialized.

The trout lives in cool running water and some varieties can live in no other, while some fishes have become accustomed to warm, stagnant water and cannot live with the trout. What is true in this respect of fishes is true of land animals as well. The large, ponderous, slow-moving reptiles of the Triassic, Jurassic and the Cretaceous, and the large mammals of the Tertiary and Quarternary could not exist except under the peculiar conditions of that time, and sooner or later had to give way to the smaller, more active and more resourceful animals of their class.

In tracing the history of fishes from their earliest existence to the present one is struck with the myriad forms he finds. It would seem that all possible effort was made by them to modify their structure to suit their environment; when this changed all their efforts came to naught, and they were destined to give way to the more favored kinds.