. The chordates. Chordata. 368 Comparative Anatomy—Its History, Aim, and Method. Fig. 292. Convergence. (Above) The pterosaur, an ancient reptile, and the hat, a later mammal, acquired similar adaptations for flight. (Below) The ichthyosaur, an ancient reptile, and cetaceans, later whalelike mammals, acquired similar adaptations for aquatic life. tions to aquatic life. They might reasonably be supposed to be more or less closely related, particularly the whale and sea cow with their similar finlike pectoral flippers, with only internal vestiges of pelvic appendages, and with similar horizontal
. The chordates. Chordata. 368 Comparative Anatomy—Its History, Aim, and Method. Fig. 292. Convergence. (Above) The pterosaur, an ancient reptile, and the hat, a later mammal, acquired similar adaptations for flight. (Below) The ichthyosaur, an ancient reptile, and cetaceans, later whalelike mammals, acquired similar adaptations for aquatic life. tions to aquatic life. They might reasonably be supposed to be more or less closely related, particularly the whale and sea cow with their similar finlike pectoral flippers, with only internal vestiges of pelvic appendages, and with similar horizontal tail-fins. But the evidence from all sources—internal anatomy, development, and paleontology— indicates that the aquatic adaptations of these animals go back to no mammalian common ancestor. Sea cows trace back to early hoofed animals. Seals and sea lions are carnivores, remote relatives of dogs and cats. There is some evidence that even the seals and sea lions have had separate origin, the "earless seals" (, without external ears) having possibly come from some otter-like ancestor, and the "eared seals" (sea lions and fur seals, having small external ears) being derived from bears. Figure 293 shows the close similarity, as regards external fea- tures, of a shark, an extinct aquatic reptile, and an aquatic mammal. But these similarities cannot be due to any "relation of genetic de- scent" because the immediate ancestors of the ichthyosaur must have been land reptiles and those of the porpoise were land mammals. Convergence has produced snakelike animals of several sorts. In addition to snakes there are the legless lizards, in external features closely resembling snakes, and certain tropical burrowing amphibians (caecilians) which, at first sight, might be mistaken for small snakes. There are many instances, too, in which the convergence is restricted. Please note that these images are extracted from scanned page images that may have been
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