45© EVOLUTIONARY MORPHOLOGY 



such as the Petrels, Penguins, Divers, Cormorants, Flamingos, 

 Ducks, and Gulls. 



In the Penguins, in addition, the wings have been modified 

 into paddles or flippers and the birds can no longer fly. It is 

 worth noticing that these aquatic birds, with the exception of 

 the penguins, have their powers of flight in no way impaired 

 by the adaptation of webbed feet ; this is a consequence of 

 the avine structure of the wing which leaves the hind limbs 

 unencumbered. A peculiar modification which may occur is 

 the absence of the 5th flight-feather carried on the ulna (5th 

 secondary remex). This condition is known as aquinto- 

 cubitalism or diastataxy, and it occurs sporadically in some 

 groups and not in others, or even in some members and not in 

 others of the same group. The significance of this modifica- 

 tion which is so peculiar is a mystery. 



The Palaeognathae include the Struthiones (Ratites) or 

 " flightless birds " and the Tinamus. In the former the flight- 

 feathers have lost the hamuli on the barbules, so that the vanes 

 are no longer resistant to the air. This character is associated 

 with the loss of the power of flight on the part of these birds, 

 and accounts for the well-known structure of the ostrich plume. 

 Also connected with their flightless condition, the Palaeo- 

 gnathae (again with the exception of the Tinamus) have lost the 

 keel on the sternum. These characters are specialisations and 

 losses from a more primitive type of flying bird. At the same 

 time the palate of the Palaeognathae is more primitive than that 

 of Neognathae, and some preserve primitive characters such as 

 the claws at the ends of the fingers in the wing in the ostrich. 

 In addition to the ostrich (Struthio, South African and Arabian), 

 the Struthiones include the Rhea (South American), the Emu 

 and Cassowary (Australasian), and the Kiwi (Apteryx, New 

 Zealand). The extinct Moa of New Zealand had reduced its 

 wings altogether. 



It is interesting to note that some Neognathae also lost the 

 power of flight, such as the extinct Dodo of Mauritius, and 

 the Solitaire of Rodriguez. Flightless birds, whether Palaeo- 

 gnathae or Neognathae, are more or less restricted to the southern 

 hemisphere or to islands, where there is little competition to 



