I think we may go farther, although in this latter consideration we may very possibly be mistaken. We have already seen that the progress made by any animal may be measured more or less accurately by the length of time during which its ancestors maintained a swimming life. The ancestors of the coelenterates settled to the bottom first. Then successively those of flatworms, mollusks, annelids, and crabs. All this time the ancestors of vertebrates were swimming in the water above. Food was probably more abundant, certainly more easily and economically obtained by a creeping life, on the bottom. But thither the vertebrate could not go. There his mail-clad competitors were too strong for him. Those which settled and tried to compete in this sort of life perished. We may have to except the ascidia, but they paid for their success by the loss of nearly all their vertebrate characteristics. The future progress of vertebrates depended upon their continual activity in the swimming life. And they were forced by their environment to maintain this. Otherwise they might, probably would, never have attained their present height of organization. Certainly at this time you would have found it hard to believe that the victory was to fall to these weaker and smaller vertebrates. Let us come down to a later period. Reptiles, mammals, and birds are struggling for supremacy. Of the power and diversity of form of these old reptiles we have generally no adequate conception. The forms now living are but feeble remnants. There were huge sea-serpents, and forms like our present crocodiles, but far more powerful. Others apparently resembled in form and habit the herbivorous and carnivorous mammals of to-day. Others strode or leaped on two legs. And still others flew like bats or birds. They were terrible forms, with coats of mail and powerful jaws and teeth. And they were active and swift. When we look at them we see that the vertebrate, though slow in gaining the lead, is sure to hold it. The internal skeleton gave fewer advantages at the start; its greatest superiority had lain in future possibilities. But which vertebrate is heir to the future? It would have been a hard choice between reptile and bird. I feel sure that I, for one, should not have selected the mammal, a small, feeble being, hiding in holes and ledges, and continually hard put to it to escape becoming a mouthful for some huge reptile. And yet the persecution, the impossibility of contending by brute strength, may have forced the mammal into the line of brain-building and placental development. The early development of mammals appears to have been slow. Palæontology proves that they were long surpassed by reptiles and birds. But the little mammal had the future. The battle was to go against the strong.