travers] STRUCTURE AND HABITS OF THE BIRD 401 



Why have these birds become extinct ? Because, as in the 

 case of the Hesperonis, they were unable to adapt themselves to 

 •changing environment. Chapman says: "The measure of a 

 bird's success in life is determined not alone by its powers as a 

 migrant (if it be migratory) ; its attractiveness when wooing a 

 mate; its skill as a nest-builder; its devotion and courage as a 

 parent; the nature of its physical and mental endowment, or the 

 degree of its intelligence, but also by the extent of its adaptability 

 and the character of its temperament." That many birds have 

 succeeded in life is proved today, when we see that they have con- 

 quered the dangers of a million years and have gained the foremost 

 rank in the scale of living creatures. 



The first thing to consider in our attempt to understand how it is 

 that birds have succeeded so well, is the general framework of the 

 body. One very interesting variation from our own bodies is 

 seen in the vertebrae. In birds there are saddles on the vertebrae, 

 a fact characteristic of birds alone. Sparrows have fourteen neck 

 vertebrae, swans have twenty-three. In all birds but the Screamer 

 there is a small bony projection from near the center of the upper 

 part of each rib, a feature found elsewhere only in crocodile-like 

 reptiles. The sternum is one of the largest bones in the bird's 

 body. Its posterior edge is of many shapes and so is of value in 

 •classification. The size of the keel of the sternum is the criterion 

 of the flying powers of the bird. A bird that trusts to the wind 

 and soars considerably has a relatively small keel. In the hum- 

 ming bird, it is said that the muscular energy is greater and the 

 bones more powerful than in any other animal and the keel of the 

 sternum is relatively of great size. Birds are able to move their 

 wings strongly and rapidly by means of well-adapted coracoid 

 bones. In the bird's wrist there are but two free bones, and the 

 femur is relatively small, the extra length being attained in the 

 •elongation of the next two lower joints. The knees of the bird 

 are generally concealed beneath the skin. In the bird embryo 

 there are several small bones in the tarsus of the leg. The skull of 

 birds and reptiles hinges on but one bony projection, while in other 

 mammals' there are two such projections — another proof of the 

 kinship of reptiles and birds. Moreover, in the embryonic bird 

 the skull is cartilaginous as it always remains in the shark; also 

 in the embryo there are four gill arches, one of which later entirely 

 disappears. Nine air-sacs are distribured through the bird's 



