1 86 NATURAL SCIENCE. March, 1895. 



Since the days oi A vchaoptevyx, its descendants, or the descendants 

 of its near relatives, have been evolved into the modern bird, which, 

 in its more perfect forms, is a perfect biped and a powerful flier ; and 

 a brief enumeration of some of the chief changes involved in that 

 evolution will throw an additional light upon my contention as to the 

 comparatively small power of flight, and as to the quadrupedal loco- 

 motion of AvchcEoptevyx. The shoulder has shifted backwards, the 

 trunk has become shorter, bringing both elbow and knee-joints nearer 

 the centre of gravity ; thereby rendering balance in walking or 

 standing independent of the aid of the fore-limb. The fore-limb, in 

 accordance with this release from one of its functions, has lost the 

 digits I, II, and III on which that function depended, and has thus 

 been reduced in inertia, while the pelvis has become widened to 

 allow for the backward displacement of the abdominal viscera, and 

 the tibia has acquired a cnemial crest of much greater size, enabling 

 the hind-limb to support the weight of the whole body, with the knee 

 flexed and placed in such position that the centre of gravity is behind 

 it. The aeroplanes of the tibiae have been abolished, and that of the 

 tail shortened, lightened, and reduced to a steering apparatus pure 

 and simple. The wings have gained in size ; their inertia has been 

 reduced by the development of air-cavities in the wing-bones ; more 

 powerful muscles have been evolved for the movement of the wings, 

 and this has involved the development of a pectoral crest and of a large 

 sternum. The reduction in weight of the head (especially jaws) has 

 aided in the backward shifting of the centre of gravity. The develop- 

 ment of the large sternum has facilitated the evolution of a respiratory 

 apparatus of more efficient type, and the shortening of the whole 

 body and concentration of its weight near one point, aided by the 

 stiffening of the back — often by ankylosis of the vertebras and always 

 by the increase in length of the functional " sacrum " — has rendered 

 the support of the hinder part of the body by cumbrous aeroplanes 

 unnecessary. 



This is by no means all that might properly be urged in favour 

 of the views I have expressed, but I fear to make my discussion too 

 long. 



C. Herbert Hurst. 



(To be continued.) 



