274 C. H. HURST, PH.D. 



joint. A distinct tubercle, for the insertion of the flexor tendon, is 

 recognisable at the proximal end of almost every one of the eighteen 

 phalanges, and these, together with the curvature of the bones, show 

 that flexor muscles were well developed and active and useful, and, in 

 view of the forms of the joints, were useful in producing extensive 

 flexion of those digits. One of these joints has been referred to by 

 some who had not seen the specimen as possibly a fracture (the joint 

 between the second and first phalanges of the third digit, marked 16 

 in Plate XV) ; but a more perfect joint does not exist in the toe of any 

 existing bird than that joint. It is perfectly preserved, and nobody 

 who has seen it can doubt for a moment that it has been evolved 

 under the influence of Natural Selection, and was exceeding well- 

 adapted to allow of a very extensive flexion. 



The feathers are as perfectly adapted to resisting the passage of 

 air through them as in any modern bird. Those who have studied 

 the mechanism of flight in detail will recognise why those feathers are all 

 so curved that the dorsal surface of the wing when at rest is convex ; why 

 the anterior division of each vane is narrower than the posterior one, 

 and is strongly curved and overlaps the posterior division of the vane 

 of the next feather in front. They will know that such a wing is 

 useful only if adequately supported by rigid bones capable of resisting 

 very considerable torsional stress. 



These two sets of structures — the digits I, II, and III, and the 

 feathers — have been evolved under the direction of Natural Selection. 

 They are both, therefore, fitted to perform the functions they actually 

 did perform. The digits were useful for some purpose involving 

 extensive flexion : they were, in fact, used to grasp parts of trees — for 

 I shall show later that the animal did not habitually walk on the 

 ground on all fours. They could not do this if those large feathers 

 were attached to them. Mr. Pycraft has shown that in Opisthocomus 

 the young use the digits for climbing, and that in order to enable 

 them to do so the development of the mid-digital and ad-digital quills 

 is delayed till such time as the young are able to fly or to climb 

 without the help of these digits. I thank him for this excellent 

 illustration (Nat. Sol, vol. v., pp. 355 and 358). It confirms the 

 opinion I have expressed that flexible digits cannot be used for 

 climbing if they bear large quills. 



