As would be expected, the greatest modifications of structure in the adaptation of air-breathers to water life are found in the limbs. No other parts of the body have such different functions in water and on land as the limbs and fins. The limbs of a dog, or a cat, or a man are feeble organs for swimming in comparison with the fins of a fish, and if the land animal must compete with fishes to prey upon them for food it must acquire like swimming powers. As a matter of fact, the limbs of all typically aquatic air-breathing animals have lost nearly all external resemblance to the legs of walking and running animals, and have become more or less fin-like in function—fin-like in shape and function, but never fin-like in actual structure. No creature can go back and begin over again, any more than a man can again become a child with all its possibilities for improvement and development. If an animal cannot modify the organs it already possesses so as to adapt them to new and changed uses by the aid of evolutionary forces it must fail in the struggle. It can never acquire new material, never get new fingers and toes, new organs or parts of organs; all its possibilities lie in the improved and new uses it can make of the material which it received from its ancestors. The beginning of aquatic adaptation of the limbs lies in the membranous webs between the toes of frogs, salamanders, ducks, seal, otters, etc., where the feet are used largely or entirely for propulsion through the water, in the absence of a propelling tail. And this membrane, in the majority of cases, is the extent of aquatic adaptation in air-breathing animals. In those animals, however, such as most of the reptiles described in the following pages, where the tail has developed as the propelling organ, the limbs lose to a greater or less extent their propelling function and become merely organs of equilibration and control. Of the two pairs of fins of fishes it is evident that the anterior ones have the more important equilibrational function; the hind ones have a much less important use as guiding organs; as a matter of fact, in not a few fishes the hind or pelvic fins have actually migrated forward to supplement the function of the pectoral fins. It is for these reasons that those animals best adapted of all for life in the water—the whales and sirenians—have lost the hind legs completely. In other tail-propelled air-breathers the hind legs have become progressively smaller and less powerful than the front ones. In all short-tailed water animals, however, where the legs, and especially the hind legs, have the important function of propulsion to subserve, they still retain the large size and firm connections with the body, examples of which will be seen in the seals, sea-otters, marine turtles, and plesiosaurs.