A fish through long ages of evolution has become well adapted to its environments; its shape is the best for speed or varied evolutions in the water; its teeth and mouth-organs are best suited for the food it requires. Now it is evident that if animals of very different habits and form should go back to the water and seek to compete with creatures already well adapted to their surroundings, they must, so far as possible, acquire like forms and like habits. And any improvement on such forms and habits that their higher development permits them to attain will of course be of advantage in their competitive struggles. A fish makes most use of its tail fin for propulsion. It follows that a land animal seeking to compete with it under like conditions must acquire a tail fin or some other organ which subserves its purpose as fully. The body fins are of little use to a fish, save for equilibration, for preserving its position, for stopping quickly, or for changing the direction of its movements quickly—very different functions from those of the corresponding organs, the limbs, of higher vertebrates. There are few better examples of predaceous, fish-eating fishes than the common gar-pike of our rivers, fishes with a slender body covered with very smooth scales, a strong tail, a short neck, and long jaws armed with numerous slender and sharp teeth. Such a fish, darting into a school of smaller fishes, by quick, sudden changes of movement, actively opening and closing its jaws, is sure to seize some of its sought-for prey. In a direct trial of speed with its victims it would most likely be worsted. There have been many animals of high and low rank which in the past and present have gone back from a terrestrial existence to a life in the water, finding at last a congenial home away from the shores. Or, perhaps, like the monitor lizards of today, they have found temporary safety in the water when hard pressed by their land enemies, and finally found, not only protection, but an abundant supply of easily obtainable food therein. As in every vocation of life there have been many failures in such attempts, many partial successes only. But not a few have found abounding and enduring success and final prosperity—success that has led possibly to undue adaptation to surroundings, and to the acquirement of great size, for that has been the invariable end of water air-breathers of long duration—specializations which finally prevented them from meeting new exigencies. It seems to be a law of evolution that no large creatures can give rise to races of smaller creatures; and as we shall see, the largest sea-animals have been the final evolution of their respective races.