Epilogue: What Comes of Studying Vertebrates 



801 



lines of specialization repeats itself, not only along the three major 

 lines of adaptation — aquatic, terrestrial, and aerial — but also along 

 various minor lines such as lead, for example, to fossorial and ar- 

 boreal adaptations. Many mammals, reptiles, and amphibians bur- 

 row. Some birds (motmots, todies; some owls, kingfishers, and swal- 

 lows; and many petrels) inhabit burrows either dug by themselves or 

 formerly the homes of other animals. Even some fishes (e.g., Pro- 

 topterus, the African, and Lepidosiren, the South American, lungfish) 

 during the dry season burrow into sand or mud at the bottom of the 

 water. And many human primates spend most of their lives in cellars, 

 mines, or tunnels. In the world's trees are most birds, but also monkeys, 

 squirrels, sloths, anteaters, opossums, and other mammals, many 

 lizards and snakes, some frogs and toads, and upon occasion may be 

 found an Asiatic climbing perch 6 feet up the trunk of a palm, and 

 gobies taking an airing among the mangroves while the tide is out. 



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Fig. 607. Repetitive radiation. The heavy arrows indicate the main line of 

 vertebrate evolution: from sharklike fishes (at middle bottom of figure) to bony 

 fishes, then amphibians which emerged onto land and went on to terrestrial reptiles, 

 thence along one line to terrestrial mammals and along another to flying reptiles 

 with feathers — birds. 



The light unbroken arrows indicate lines of specialization which are off the 

 main line of evolution: e.g., from crossopterygians to teleost fishes, some of which 

 make excursions ashore (climbing perch, Anabas scandens) and others essay flight; 

 from terrestrial reptiles to flying reptiles without feathers (pterosaur); and from 

 terrestrial mammals to flying mammals. 



The broken arrows indicate reversion to ancestral habitat: aquatic ichthyosaurs 

 and turtles from terrestrial reptiles; whales from terrestrial mammals (the "con- 

 necting link" in the figure is hypothetic); flying birds to terrestrial birds (ostrich) 

 and amphibious birds (penguin). 



The figure should have included flying frogs, and men in automobiles, sub- 

 marines, and airplanes. Net result in our present world: all are everywhere, in 

 water, on laud, and in air. 



