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" (i.e., 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 



