BIRDS 131 



several things otherwise obscure would be explained. The 

 rapid beating of the bony knobs against one another, the 

 knobs being covered with horny skin or cartilage or some 

 such covering, would produce exactly the kind of noise 

 indicated by the above descriptions, a rattling or rumbling 

 noise, similar perhaps to that of a kettle-drum, or of the 

 " bones " of a negro orchestra. Secondly, the wings would 

 naturally meet, not only at a particular spot on the meta- 

 carpal, but along a greater length of the edge in the wrist 

 region, and thus we can understand the exostosis on the 

 distal end of the radius, and the greater size of that bone. 

 Thirdly, the wings would meet edge to edge at the carpal 

 region, with a slight inward direction, and that is the 

 precise position of the exostoses. It is obvious enough how 

 thoroughly this hypothesis agrees with the principle by 

 which I explain the evolution of such structures. It is the 

 case of the stag's antler over again. The bones are violently 

 hammered, and the irritation causes a kind of hypertrophy, 

 which may in a sense be considered pathological, but which 

 in the course of generations becomes normal, because con- 

 stant and hereditary. Some may be inclined to maintain 

 that even if my suggestions are correct, the exostoses 

 were not hereditary, but were entirely due to irritation 

 in the individual. That in my opinion is very improb- 

 able. 



The birds, doubtless, used their exostoses as weapons in 

 fighting, and perhaps struck with the wing in the same way 

 as when beating the wings together. In the bones ex- 

 amined evidence of reunited fractures of the radius and 

 ulna have frequently been seen, and these are considered to 

 show the "effect of the cestus-like armature of the wing" 

 in fighting. Probably this is true, though the bones may 

 have been sometimes broken in the beating of the wings 

 together, for the descriptions indicate that the whirling and 



