5 i6 EVOLUTION OF BIRDS xvm. 6- 



common ancestry with the petrels. Unlike most other water-birds they 

 swim chiefly by means of the fore-limbs, modified into flippers; the 

 feet are webbed. The penguins are mainly confined to the southern 

 hemisphere. They come ashore to breed; many make no nests, but 

 sometimes carry the one or two eggs on the feet throughout the 

 incubation period. The emperor penguin breeds in winter on the 

 Antarctic ice and is the only bird that never comes on land. The egg 

 is supported on the feet. 



7. Modern birds. Superorder Neognathae 



All the remaining birds have the characteristic palate, sternum, and 

 other features already described and are placed in a single group as the 

 superorder Neognathae. Birds of this type probably existed in the 

 Cretaceous and many of the orders are known from Eocene times, but 

 the fossil evidence is not adequate for us to be able to say when they 

 became numerous and differentiated as they are now. The existing 

 birds, as has already been suggested, show very great variety of details 

 of structure and habits, superimposed on a common basic plan. 

 Classification of the vast number of genera involves recognition of 

 over forty distinct orders and even then one of the orders, the Passeri- 

 formes, contains about half of all the species. Unfortunately, little can 

 be done in a short space towards describing the great variety of bird 

 life. We can only list the important orders, mentioning a few of the 

 characteristics of some of the more interesting types. Birds are so 

 conspicuous that their species have been very fully described, there 

 are about 25,000 well-defined species and subspecies. The arrange- 

 ment of the orders adopted for the survey, Birds of the World, by 

 J. L. Peters has been used here. 



Order 1. Gaviiformes. Loons 



The divers are aquatic birds retaining some primitive characteristics. 

 They are birds of open waters, feeding mainly on fishes. Various 

 species of Gavia live mostly on the sea, but breed by lakes throughout 

 the holarctic region. 



Order 2. Colymbiformes. Grebes 



The grebes (Colymbus = Podiceps) (Fig. 303) are also aquatic birds, 

 almost unable to walk on land. They resemble the divers in some 

 ways, but are perhaps not closely related to them. They nest on lakes, 

 laying a small number of white eggs in a floating nest. 



