72 STRUCTURE OF THE VERTEBRATES 



in the course of evolution. This fact is of interest as no known 

 living bird has any sign of teeth, the bill being made of thick- 

 ened epidermis like that of the claws. The so-called "egg tooth" 

 of the newly hatched bird is an epidermal growth of the bill 

 and is lost soon after the embryo pecks its wa}^ out of the shell. 



The birds of today are divided into two great groups showing 

 two distinct lines of evolution: (1) the flying birds which have 

 a keel (carina) on the breast bone, and (2) the running birds 

 with degenerate wrings and a smooth breast bone. The latter un- 

 doubtedly evolved from the former, but for ease of discussion 

 will be considered first. 



Ratite birds are those with smooth breast bones, including 

 several extinct species and the living ostrich, rhea, cassowary, 

 emu, and the kiwi. The roc of Sinbad the Sailor belonged to 

 this group, and was an enormous bird which laid an equally 

 enormous egg. As the skeleton and egg only were known, it was 

 easy for the layman to draw the conclusion that it would have 

 great powers of flight, despite the fact that it was a non-flying 

 species. 



The ostrich is a native of Africa, but the animal has been 

 naturalized in many of the warmer regions of the earth, the 

 plumes from the farm animals being of better quality than those 

 taken from wild birds. It stands about eight feet high, with small 

 wings and powerful legs. The egg has the content of about two 

 dozen hens' eggs, and is the largest single cell known. 



The rhea is South American in distribution. The emu and 

 cassowary inhabit Australia, and like the former, occupy open 

 grassy plains. The Kiwi or Apteryx (without wings) is the most 

 specialized of the ratites, and is confined to New Zealand. The 

 wings are small boned and are not visible externally. For their 

 size, these birds lay huge eggs, each being approximately one- 

 fourth of the body weight. 



Carinate birds are world-wude in distribution. In size they 

 range from the smallest humming bird to the albatross and con- 

 dor. In flying ability they vary from the non-flying penguins to 

 the great speeds attained by migratory birds. Modern airplanes 

 have demonstrated speeds of more than a hundred miles an 

 hour in some birds, but whether this could be maintained for 

 long periods is not proved. 



