798 Comparative Morphology of Chordates 



massive salivary glands which are definitely localized and physio- 

 logically diversified. 



Repetitive Radiation 



One of the most characteristic properties of "Life" is its pro- 

 pensity for increase. Organisms grow — both in ontogeny and in 

 phylogeny they increase in size and differentiation. They multiply in 

 number, expand their habitats, push themselves along their ramifying 

 lines of specialization into every spot which can, by any trick of adap- 

 tation, be made habitable. The vertebrates vividly illustrate this. 

 Fishes, already widely dispersed in the world's waters, produced a line 

 of descendants which went ashore and achieved an amphibious life. 

 Some "amphibians" became wholly terrestrial. Reptiles and mammals 

 began their careers as land animals, but in the course of time some 

 members of each class reverted to amphibious or even wholly aquatic 

 life, retaining lung-breathing and, in the case of reptiles, the necessity 

 of breeding on land or else becoming viviparous. The ichthyosaurs 

 may have been viviparous. Along other lines of specialization, reptiles 

 went aloft and achieved flight as pterosaurs or as the much more 

 highly specialized birds. Mammalian bats likewise invaded the air. 

 Having acquired a highly efficient mechanism of flight, some birds 

 came down to earth, suffering reduction of wings, and became ostriches, 

 kiwis, dodos, and various other flightless birds. The extreme of rever- 

 sion to ancestral habitat occurred when birds, descended from fishes 

 via amphibians and terrestrial reptiles, resorted to aquatic life and, 

 in such birds as penguins, became in some respects the avian equiva- 

 lents of fishes. 



Typically, fishes are aquatic, amphibians are amphibious, reptiles 

 and mammals are terrestrial, and birds are aerial. Rut it is not strain- 

 ing the truth too much to say that each of the five groups has some 

 representative on all three of earth's habitable levels, water, land, and 

 air. 



Flying fishes, using the greatly expanded pectoral fins as wings, are 

 better fliers than some flying birds. Some fishes are capable of loco- 

 motion on land. Certain small fishes (e.g., the "mummichog," Fundu- 

 lus) , if left in a pool as the tide recedes, will travel, by flapping motions 

 of the body, across the beach and back into the ocean. In southern 

 Asia is a small fish (about 8 inches long), the climbing perch (Anabas 

 scandens), which is able to make long sojourns out of water. Compli- 

 cated expansions of certain of the visceral arches greatly increase the 

 surface of the subopercular space. These expansions, covered by a 

 highly vascular membrane, provide for aerial respiration while the 

 fish is out of water. According to accounts which seem to be trust- 



