226 PEOF. W. K. PAEKEE ON THE 



metacarpals, on the extensor side, a semi-elliptical cartilage has appeared (me. 2 '), and, 

 below, a flange of cartilage has grown from the ulnar margin of the proximal phalanx of 

 the index, or 2nd digit (dg. 2 ) ; this is not very distinct in the Chick ; it is quite distinct 

 and formed later than the main phalanx in many birds. These parts, to say nothing 

 here of others that appear in other kinds of birds, seem to me to be atavistic remnants 

 or vestiges of an archaic poly dactyle or many -rayed fin. That these parts have a some- 

 what teleological importance does not help in their interpretation ; a very slight peri- 

 osteal growth from the normal parts would have sufficed for this ; there was no reason, 

 from this standpoint, for the appearance of additional parts — parts that appear some- 

 what late, and lose their independence early, and therefore, as they cannot be accidental, 

 must have a morphological meaning ; that meaning I have suggested to be atavism. 



But there are so many other unexplainable parts in the wings of Birds, that it will be 

 necessary for me to devote a whole and not a small part of a memoir to this subject. 

 The Common Fowl is, after all, only a sort of initial or tentative bird, as to the matter of 

 flying ; it just serves as an introduction to the Carinatae. 



XI. — The Hip-girdle and Hind Limb in the Chick. 



The fixed swinging-point of the hind limb, or hip-plate, is not only of extreme interest, 

 morphologically, as forming part of a bird, but Palaeontology thrusts itself in here, so 

 to speak, and half the beauty of the piece of the framework is missed unless we become 

 comparative anatomists as well as ornithotomists. The hind-quarters of the Bird and 

 the Iguanodon are modified in a similar manner, as is well known ; and this has led to 

 too great an approximation of these extremely different types in the classification of the 

 Sauropsida — extinct and recent. We know, now, that the peculiar rotation backwards of 

 the pubes and ischia, in the Bird, like that in the Iguanodon, takes place during the early 

 growth of the skeleton (see Miss Alice Johnson's paper, Stud. Morph. Lab. Camb. vol. ii. 

 plate 5). 



I have recently shown the same changes in the Duck (Mem. Boy. Irish Acad. 1890). 

 Of course we can only feel certain that the same change took place in the pelvis of the 

 Iguanodon in its early growth. This, indeed, is one of the difficulties in comparing those 

 huge extinct Ornithoscelida with the developing bird ; and rash, impatient Biologists 

 are always taking it for granted that the structure of these extinct forms is in a 

 primitive state; it is nothing of the kind — any great changes, both in number and in 

 form, must have occurred in the egg, and afterwards in the young, of those types whose 

 structure we only know in the adult state. 



In my first stage of the Chick the pelvis (Plate XXII.) corresponds in form with that of 

 an adult Tinamou (T. Z. S. vol. v. plate 39) ; but in the Duck, Anas boschas (domesticus), 

 the embryo at the end of the 1st week is not so much advanced ; and the pubis and 

 ischium diverge from each other at a large angle (as in Miss A. Johnson's figure of the 

 Chick, Stud. Morph. Lab. Camb. vol. ii. plate 4. figs. 3-6); and whilst the ischium grows 

 downwards and backwards, the pubis grows downwards and forwards, exactly as in the 



