Every trait of vertebrates is a promise of a great future. Its internal skeleton gives it the possibility of large size. This gave it in time the victory in the struggle with its competitors, as to whether it should eat or be eaten. It is vigorous and powerful, for all its organs are at the best. It gives the possibility of later, on land, becoming warm-blooded, _i.e._, of maintaining a constant high temperature. It is thus resistant to climate and hardship. In time its descendants will face the arctic winter as well as the heat of the tropics. But it has started on the road which leads to mind. The greater size is correlated with longer life. The lessons of experience come to it over and over again, and it can and must learn them. It is the intelligent, remembering, thinking type. The insect had begun to peer into the world of invisible and intangible relations, the vertebrate will some day see them. This much is prophecied in his very structure. He must be heir to an indefinite future. * * * * * You have probably noticed that the vertebrate differs greatly from all his predecessors. The gulf between him and them is indeed wide and deep. His origin and ancestry are yet far from certain. But an attempt to decipher his past history, though it may lead to no sure conclusions, will yet be of use to us. Practically all aquatic vertebrates lead a swimming life, neither sessile nor creeping. The embryonic development of our appendages leads to the same conclusion. We must never forget that the embryonic development of the individual recapitulates briefly the history of the development of the race. Now the legs and arms, or fore- and hind-legs, of higher vertebrates and the corresponding paired fins of fish develop in the embryo as portions of a long ridge extending from front to rear of the side of the body. This justifies the inference that the primitive vertebrate ancestor had a pair of long fins running along the sides of the body, but bending slightly downward toward the rear so as to meet one another and continue as a single caudal fin behind the anal opening. Such fins, like the feathers of an arrow, could be useful only to keep the animal "on an even keel" as it was forced through the water by the lateral sweeps of the tail. They would have been useless for creeping.