118 ON THE TRAIL OF AN EXTINCT BIRD, 



Proportions : The ulna being in correlation with the rest of the 

 wing bones, and, in conjunction with them, determining to some 

 extent the shape of the complete organ, and this again being in 

 relation with the volant activity of the bird, we might expect to 

 be able to recognise a correspondence between the proportion of 

 the bone and the bird's habits of flight ; and in certain groups, as 

 the petrels, swifts, and eagles, whose livelihood depends on 

 continuous exertion of wing-power, we find that such a relation 

 does exist. In the soaring birds there is a notable slenderness of 

 the ulna, accompanying an elongation and narrowness of the wing, 

 which we may conceive to be necessary to sustained buoyancy 

 upon and rapid evolution in moving air ; and had adaptation 

 persisted in being the sole factor in the formation of the wing the 

 task of placing an unknown bird amongst its kindred, as 

 determined by their powers of flight, would have been compara- 

 tively easy. But it is clear that teleology may be at fault. A 

 similar tenuity of the ulna is found in birds whose flight is not 

 habitually sustained, though on occasion it may be long and rapid 

 — for example, in storks, swans, and pelicans; nay even in others, 

 as the giant kingfisher, whose wings serve only for short and 

 laboured flight. Looking round for a solution of the difficulty, and 

 seeing the prevalence of long necks in the birds last mentioned, 

 we are for a moment tempted to abandon adaptation as a cause 

 and suppose their long ulnas to be due to correlation of growth ; 

 but even this somewhat violent assumption would be illegitimate, 

 seeing that plovers and sandpipers, with long ulnas, have short 

 necks, while most ducks have, with long necks, short ulnas. The 

 only plausible explanation seems to be offered by heredity. 

 Though forbidden to account for the long ulnas of many existing 

 birds by attributing them to adaptive modifications, we are 

 permitted to conceive that they have been handed down from 

 ancestral forms whose modes of flight required them, and retained 

 by the prepotency of heredity over adaptation. If it be said that 

 heredity as thus used is a convenient harbour of refuge for 

 ignorance, be it so until we know better. 



