Sauropsida: Class Aves 557 



between Hesperornis and the dodo. The attainment of bulk and 

 weight so great as to be incompatible with good flying was associated 

 with terrestrial living in the dodo and with aquatic living in Hesper- 

 ornis — and both suffered extinction. The flightless cormorant, too, 

 is the largest of the cormorants. 



It is certain that these adaptations for terrestrial or aquatic living, 

 accompanied by reduction or loss of capacity for flight, must have had 

 origins widely separated from one another in time, space, and genetic 

 relation. The modern orders of birds have become differentiated since 

 the Cretaceous. The aquatic modifications of Hesperornis, the ant- 

 arctic penguin, and the great auk of the North Atlantic cannot be of 

 common genetic origin. The dodo, a flightless pigeon-like bird, is no 

 more closely related to ostriches than are flying pigeons. Flightless 

 rails must have been derived from flying rails, Harris' cormorant from 

 flying cormorants. The "steamer duck" of southern South America is 

 an especially large seagoing duck which, while young, is able to fly, but 

 whose body eventually acquires size and weight disproportionate to 

 the spread of wings, so that flight becomes impossible. The wings, how- 

 ever, are used as paddles as the bird floats on the water — a mode of 

 propulsion suggesting the old-fashioned "side-wheel" steamer. This 

 case is significant in that, within the lifetime of the individual animal, 

 a flightless bird is derived from a flying bird. The young bird is a typical 

 duck but the adult, in being flightless and using the wings as paddles, 

 simulates a penguin. 



Both the reptiles and the birds strikingly exhibit that common 

 propensity of living things for such adaptive diversification as enables 

 them to occupy environments of maximum extent and variety. Rep- 

 tiles were primarily land animals but, in the course of time, many of 

 them assumed aquatic life and a few became flying reptiles. Other 

 descendants of reptiles became flying birds but, in the course of time, 

 descendants of flying birds reverted to terrestrial living or assumed 

 aquatic living and became flightless, yet retaining so many of the basic 

 anatomic peculiarities of their flying ancestors that we cannot regard 

 them as anything but birds. 



In these terrestrial and aquatic adaptations, birds show numerous 

 and striking instances of convergence in evolution. Flightless ter- 

 restrial birds can persist only in an environment where large carnivo- 

 rous mammals are few or lacking. Of all localities, New Zealand, an 

 island quite devoid of native carnivorous mammals, is richest in flight- 

 less birds, some aquatic and many terrestrial, representing various 

 Orders. 



