48 THE BIOLOGY OF BIRDS 



Birds. Architectural experts say that the beams of this 

 framework are disposed so as to give strength to the bone as 

 a whole, enabling it to stand the wonted strains and pressures. 

 Some of the most pneumatic of birds, such as the hornbills, 

 are poor fliers, and one cannot regard the pneumaticity 

 as essential to powerful flight ; nevertheless, other things 

 equal, the pneumaticity and the hollow girder architec- 

 ture must make flight easier, and may be regarded as 

 adaptive. 



Tendency to Fusion. — A second general character of 

 the bird's skeleton is the tendency that adjacent bones have 

 to fuse together while still in the making. What is behind 

 this tendency we do not know ; it must express a general 

 constitutional peculiarity, for it occurs in so many different 

 parts of the skeleton, e.g. the skull, the thoracic region of 

 the backbone in flying birds, the sacral and adjacent regions, 

 the end of the tail, the region of the wrist and hand, and the 

 ankle. It is possible to show the advantage of these fusions, 

 as we shall proceed to do, but a tendency so widespread 

 must surely rest on a constitutional peculiarity. Such 

 compound bones as the carpo-metacarpus and tarso- 

 metatarsus are absolutely distinctive of birds, never occur- 

 ring elsewhere. But a coalescence of bones sometimes 

 occurs as a pathological variation in other classes. In 

 Running Birds the bones of the skull are for a long time 

 incompletely fused, and the vertebrae of the thoracic region 

 remain free. On the other hand, in these same Running 

 Birds there is a fusion of scapula and coracoid, which 

 does not occur in Flying Birds. It may be noted that 

 while one of the methods of evolution is to multiply the 

 number of separate parts in a structure, as may be illustrated 

 by the presence of over 200 vertebrae and pairs of ribs in 

 some snakes, another method is to reduce the number of 

 separate parts, and this is seen in the tendency to fusion 

 exhibited by adjacent bones in the bird's skeleton. The 

 terminal caps or epiphyses characteristic of mammals are 

 not seen in birds. 



