546 POPULAR SCIENCE MONTHLY. 



very large yolk-sac (or which are admittedly derived from such forms). 

 Now wherever we have a large yolk-sac we have developed on its surface 

 a rich network of blood vessels for purposes of nutrition. But such a 

 network must necessarily act as an extraordinarily efficient organ of 

 respiration, and did we not know the facts we might venture to prophesy 

 that in forms possessing it any other small skin organ of respiration 

 would tend to disappear. 



"No doubt these external gills are absent also in a few of the ad- 

 mittedly primitive forms such as, e. g., (neo-) Ceratodus. But I would 

 ask that in this connection one should bear in mind one of the marked 

 characteristics of external gills — their great regenerative power. This 

 involves their being extremely liable to injury and consequently a source 

 of danger to their possessor. Their absence, therefore, in certain cases 

 may well have been due to natural selection. On the other hand, the 

 presence in so many lowly forms of these organs, the general close sim- 

 ilarity in structure that runs through them in different forms and the 

 exact correspondence in their position and relations to the body can, it 

 seems to me, only be adequately explained by looking on them as being 

 homologous structures inherited from a common ancestor and conse- 

 quently of great antiquity in the vertebrate stem. ' ' 



As to the third theory, Professor Kerr suggests tentatively that the 

 external gill, as developed in salamanders and in the young Lepidosiren, 

 may be the structure modified to form the paired limbs. Of the homol- 

 ogy of fore and hind limbs and consequently of their like origin there 

 can be no doubt. 



The general gill structures have, according to Kerr, "the primary 

 function of respiration. They are also, however, provided with an 

 elaborate muscular apparatus comprising elevators, depressors and 

 adductors, and larva? possessing them may be seen every now and then 

 to give them a sharp backward twitch. They are thus potentially motor 

 organs. In such a Urodele as Ambly stoma their homologues on the 

 mandibular arch are used as supporting structures against a solid sub- 

 stratum exactly as are the limbs of the young Lepidosiren. 



' ' I have, therefore, to suggest that the more ancient Gnathostomata 

 possessed a series of potentially motor, potentially supporting structures 

 projecting from their visceral arches ; it was inherently extremely prob- 

 able that these should be made use of when actual supporting and motor 

 appendages had to be developed in connection with clambering about 

 a solid substratum. If this had been so we should look upon the limb 

 ab a modified external gill ; the limb girdle, with Gegenbaur, as a modi- 

 fied branchial arch. 



"This theory of the vertebrate paired limb seems to me, I confess, 

 to be a more plausible one on the face of it than either of the two which 



