284 0. H. HURST, PH.D. 



development of a pectoral crest and of a large sternum. The reduc- 

 tion in weight of the head (especially jaws) has aided in the backward 

 shifting of the centre of gravity. The development of the large 

 sternum has facilitated the evolution of a respiratory apparatus of 

 more efficient type, and the shortening of the whole body and concen- 

 tration of its weight near one point, aided by the stiffening of the 

 back — often by ankylosis of the vertebrae 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, 



IV. — Answers to Critics. 



Many and various arguments have been urged against my view as to 

 the homology of the digits of the wing of an ordinary bird, as well as 

 against my interpretation of the photograph of Archceopteryx and other 

 contentions contained in an article on sources of error, which was pub- 

 lished in "Natural Science" (Vol. III. p. 275). 



Mr. Pycraft asks (p. 445), Where did my supposed but unseen 

 digits IV and V articulate in Archceopteryx ? I have answered this 

 already in the preceding pages of this article; but there is good reason 

 for answering it again. The digits did exist, and their metacarpals 

 are conspicuous in the London specimen, and they articulated to that 

 large rounded carpal bone which is shown at the distal end of the 

 radius and ulna of the right wing in Fig. 2, p. 116. 



Mr. Virgil L. Leighton, in No. hi. of " Tuft's College Studies," says 

 (p. 71), "That there is developed a fourth digit in the avian manus is 

 beyond question [!], and the fact that this comes upon the ulnar side 

 of the three permanent fingers is sufficient to invalidate the nomencla- 

 ture III, IV, and V of Hurst." The "plausibility" of my view is, 

 moreover, founded on ignorance, and " no one without a theory to 

 support would regard it [i.e., what I called os pisiforme in Nat. Sci., 

 Vol. III., p. 279] other than a digit." Mr. Leighton, of course, knows 

 that greater men than myself regard the os pisiforme itself as the 

 remnant of a digit ; but even if he did not, the alleged resemblance of 

 it to a digit in a certain stage in the development of a certain bird 

 {Sterna) would not in the least affect my conclusions. It is not diffi- 

 cult to name a few mammals in which things occur much more like 

 digits beyond the normal five than the embryonic rudiment he refers 



