ADAPTATIONS OF THE SKELETON 51 



of especial value in flying and swimming birds. In the 

 Running Bird the breastbone is a broad shield, not extending 

 far back, but the weight of the viscera in the Running Bird 

 is borne by the strongly developed dorsal and lumbar 

 vertebrae, and by the backward extension of the rib-frame- 

 work. 



It stands to reason that a keel should be absent or vestigial 

 in the Running Birds, and it is interesting to find that it 

 should be degenerate in the New Zealand parrot (Stringops) 

 which has ceased to fly and taken to burrowing. But when 

 we get beyond recording an intelligible correlation between 

 a degenerate keel and little capacity for flight, we get into 

 difficulties. Is the degenerate state of the keel in Stringops 

 the cumulative inherited result of an abandonment of 

 flying ? Or was there a germinal variation in the direction of 

 degenerate keel which led the bird to be sluggish in flight ? 

 Or, given an exploiting of a new kind of habit and habitat, 

 is the reduction of the keel a result of an economical variation 

 in the disposal of skeletal material, and of a cessation of 

 that persistent sifting which seems to be sometimes neces- 

 sary to keep a variable structure up to the level of efficiency ? 

 Few naturalists will be inclined to dogmatise as to which 

 of these answers is nearest the true interpretation, but the 

 balance of evidence at present is strongly against the first. 



(3) In adaptation to flight there is a very firm attach- 

 ment of the pectoral girdle. The lower ends of the cora- 

 coids fit into deep grooves on the anterior margin of the 

 sternum ; the upper ends are strongly bound to the front 

 end of the scapula by ligaments. The sabre-like scapula 

 is lashed by ligaments to the dorsal surface of the ribs 

 along which it lies. The glenoid cavity where coracoid and 

 scapula meet is well suited for the free action of the head of 

 the humerus ; it admits of free play for the wings. The 

 spring-like merrythought (clavicles and an interclavicle) 

 forms part of the fulcrum against which the wings work ; 

 it is bound by ligament to the front of the sternum, and the 

 interclavicle is sometimes fused to the tip of the keel. If 

 the merrythought is broken, the bird cannot fly. It is 



