There is a period of life, in which, whatever may be the final form of their organs of locomotion, whatever may be the final difference between the anterior and posterior extremities, vertebrated animals have uniform legs, in the shape of little paddles or fins. This is the case with lizards as well as birds. A robin's wing and a robin's leg, which are so different from a bat's wing and a bat's leg, do not essentially differ when young from the leg and arm of a bat. Wherever we observe combined fingers preserving this condition, we have a decided indication that such animals rank lower in the group to which they belong. This is all-important, as we are enabled at once to group animals which are otherwise allied, in a natural series, as soon as we know whether they have combined or divided fingers. And the degree of division to which the legs rise in their development is a safe guide in our classification. Look, for instance, at the legs of dogs and cats, in which the fingers are completely separated, and so elongated, that the animals walk naturally upon tip-toe, and compare them with others, bears, for instance, which walk upon the whole sole of the foot; and, again, with those of seals or bats, which remain united, and constitute either fins or a wing. There are other reasons sufficient to convince us that the order of arrangement which he had assigned them, according the development of the fingers, is justified by the state of development of the other organs of the mammalia, and especially of their higher organs and intellectual faculties and instincts. And I will also add, says Professor Agassiz, that mankind are not excluded from this connection, but, in common with other vertebrata, we are all at one stage of existence provided with paddles or fins, which are afterwards developed into legs and arms.--_American Annual of Scientific Discovery_, p. 324. 16. _The Manatus or Sea Cow, the Embryonic Type of the Pachydermata?_--Professor Agassiz thinks that the Manati have been improperly considered cetaceans: they differ from them in the form of the skull, which is elongated, and in the position of the nostrils, which are in front. On the other hand, the skull resembles that of the elephant in front (particularly when seen from above), in some of the details of the facial bones, which are not like those of the cetacea, in the palatine bones, the arrangement of the teeth, and in the curve of the lower jaw. Professor Agassiz, believed this to be the true embryonic type of the Pachydermata.--_American Annual of Scientific Discovery_, p. 313.