556 Comparative Morphology of Chordates 



clumsy proportions than the dodo, was found in a neighboring island, 

 Rodriguez, at about the same time and suffered extermination by man 

 before the end of the eighteenth century. 



On assuming aquatic life, the bird's adaptive complex becomes 

 triangular, involving relations to air, water, and land. Increase in the 

 adaptation of the locomotor mechanism to aquatic activities is at the 

 cost of facility of motion in the air or on land, or both, inasmuch as 

 swimming and diving may be effected by use of either the wings, the 

 legs, or both. It is to be noted, however, that, while the wings may be 

 vestigial or lacking, the legs, even if highly adapted for swimming, 

 must remain capable of taking the bird ashore. Birds' eggs must be 

 laid on land and must develop in air. The albatross and petrel 

 (Procellariiformes: Fig. 425) and the gulls and terns (Cha- 

 radriiformes) retain efficiency at all three points of the triangle. The 

 wings, not used in swimming, remain capable of strong and long- 

 sustained flight. The bird floats lightly and by use of large webbed feet 

 swims easily but not swiftly. On shore the legs serve well enough for 

 birds not given to hopping or running. In Anseriformes the feet are 

 more highly adapted for swimming, to the detriment of the walking 

 gait, which becomes the waddle of the duck. The wings remain unim- 

 paired as organs of flight. The loon (Colymbiformes: Fig. 424) is a 

 marvelous swimmer and diver and an especially strong flier, but very 

 clumsy ashore. In the auks (Charadriiformes) the wings, which 

 are relatively short and rounded, are used as paddles in swimming and 

 are correspondingly less well adapted for flight. The birds are slow 

 and heavy fliers. In the "great auk" of the North Atlantic, extinct 

 since about 1850, the wings had become so highly specialized for 

 swimming that they were quite incapable of flight. This condition in 

 the northern great auk finds a close parallel in the antarctic penguin 

 (Sphenisciformes: Fig. 423), whose wings are functionally flippers 

 or fins. The feathers on the wings are reduced to short, compactly 

 arranged, almost scalelike structures. In these quite flightless birds the 

 very short legs are set far back on the body and the big feet, fully 

 webbed, are used for steering. In swimming and diving these birds 

 rival fishes, but as pedestrians they are pitiably (and laughably) 

 incompetent. 



Exceptional are aquatic birds whose wings are so reduced as to be 

 capable of neither flight nor swimming, the webbed feet serving as 

 paddles. Such a bird is the flightless "Harris' cormorant" of the 

 Galapagos Islands. Another example is the extinct Cretaceous Hesper- 

 ornis (Fig. 418), a gigantic (nearly 4 feet long) loonlike swimmer and 

 diver but with only trivial vestiges of wings. There is a certain parallel 



