FINS OF ELASMOBKANCHS. 459 



Although it may seem easier and more prudent to consider the various conflicting 

 views with respect to the special homologies of piscine and non-piscine vertebrate limbs, 

 before attacking the wider question — that as to the essential nature of all vertebrate 

 limbs — I nevertheless believe that the contrary course is the one which is less likely to 

 mislead. 



Eecent experience has abundantly shown to what different interpretations, more or 

 less hastily assumed, special homologies may lend themselves. Thus even so distin- 

 guished a naturalist as Professor Gegenbaur has found it needful to change his 

 teaching upon this subject no less than three times since 1865. These repeated modi- 

 fications, however, are far from being any discredit to that gifted anatomist ; on the 

 contrary, they testify to his flexibility of mind, and to his eagerness to adopt new truths 

 rather than to adhere with pertinacity to any original view with which his name may 

 have become associated. Nevertheless such changes show that the subject is one apt 

 to mislead ; and I am the more persuaded of this, since I believe he will find it 

 desirable to make yet a fourth change, as I venture to think that even in his last view 

 he, in common with Professor Huxley, is mistaken. 



Obviously, if we can arrive at any tolerable certainty as to the nature of vertebrate 

 limbs in general, the recognition of that general nature, or fundamental condition, may 

 be expected to throw light on the structures derived from that fundamental form, and, 

 to a greater or less extent, on their special homologies. 



Vertebrate limbs may be, and have been taken to be, either parts derived (in one 

 way or another) from the axial skeleton, or else special parts which have been ap- 

 pended to and have become more or less intimately connected with that axial skeleton. 



In 1843, Oken ^ taught that arms and legs were so many liberated ribs ; and Carus, 

 following him to a certain extent, regarded them as elements radiating from the 

 exterior of a rib-like arch. 



In 1848, Professor Owen^ propounded the view that the vertebrate limbs are 

 diverging appendages attached to ribs, and serially homologous with such parts as tlie 

 uncinate processes of the ribs of birds and the branchiostegal rays of fishes. He also 

 taught that the shoulder-girdle is an axial structure made up of pleurapophyses and 

 haemapophyses. 



In 1832, Maclise^ represented the limbs as modified ribs, the parts beyond the 

 elbow and the knee, however, corresponding with the interspinous bones and fin-rays of 

 fishes' azygos fins. 



In 1857, Professor Goodsir * described the limbs as being parts of so many radiating 

 actinapophyses, actinapophyses being parts radiating outwards from the sides of the 



' Lehrbuch der Natur-Philosophie, p. 330. 



' Archetype and Homologies of the Vertebrate Skeleton. 



' Todd's Cyclopaedia, vol. iv. page 70, fig. 490. 



■* Edinburgh New Philosophical Journal, vol. v. (new series) 1857, page 178. 



