5H EVOLUTION OF BIRDS xvm. 4- 



a detailed history of the evolution of the various orders that are 

 recognized. We do not even know whether bird life first became 

 abundant after the Cretaceous, at the same time as the mammals 

 began to be numerous. 



5. Flightless birds. Superorder Palaeognathae 



The flightless birds or 'ratites', such as the ostrich, cassowary, and 

 kiwi, with reduced wings and no sternal keel, long legs and curly 

 feathers, have in the past been placed in a distinct group and regarded 

 as primitive. Indeed, it has even been suggested that they diverged 

 so early from the ancestral avian stock that they never passed through 

 a flying stage. Recently, however, de Beer and others have pointe' out 

 that certain of their allegedly primitive characters, such as the arrange- 

 ment of the palate bones, may be regarded as manifestations of neoteny 

 and do not indicate a truly primitive condition. Some neognathous 

 birds pass through a palaeognathous stage during development. The 

 evidence strongly suggests that the 'ratites' have been descended from 

 flying birds and are not a natural group, but represent several different 

 evolutionary lines. The detailed relationship of these is still obscure 

 and for convenience they are retained in a superorder Palaeognathae. 



The various ratite birds have been placed in as many as eight dis- 

 tinct orders, but the orders of the ornithologist generally represent 

 lesser degrees of difference than are usual elsewhere in the animal 

 kingdom. The ostriches (Struthio) are the largest living birds, now 

 limited to Mesopotamia. The rhea (Rhea) occupies the same ecological 

 position in South America and the emu (Dromiceius) and cassowary 

 (Casuarius) in Australasia. The moas (*Dinornis) were another type; 

 several species lived in New Zealand until recent times. The elephant- 

 birds (*Aepyornis) were similar, with several species in Madagascar 

 in the Pleistocene. Some were larger than ostriches, with eggs 

 estimated to weigh more than 10 kg, presumably the largest single 

 cells that have existed! 



The kiwis (Apteryx) of New Zealand are smaller, terrestrial birds 

 whose relationship to the ratites is doubtful. They are nocturnal and 

 insectivorous or worm-eating, with a long beak and small eyes. The 

 sense of smell and the parts of the brain related to it are better 

 developed than in other birds; it is not clear whether this is the 

 retention of a primitive feature. The palate shows large basipterygoid 

 processes. There is a penis, as in other ratites. 



Still more doubtful is the position of the tinamus (Tinamus), ter- 

 restrial birds rather like hens, of which about fifty species are found 



