234 Mr Kerr, Note on Hypotheses as to the Origin 



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 1 . 



No doubt these External Gills are absent also in a few of the 

 admittedly primitive forms such as e.g., 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 similarity 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 consequently of 

 great antiquity in the Vertebrate stem 2 - 3 . 



These structures have the primary function of Respiration. 

 They are also however provided with an elaborate muscular 

 apparatus comprising Elevators, Depressors, and Adductors, and 

 larvae 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 Amblystoma their homologues on 

 the mandibular arch are used as supporting structures against a 

 solid substratum, exactly as are the limbs of the young Lepido- 

 siren. 



I have therefore to suggest that the more ancient Gnathosto- 

 mata possessing a series of potentially motor, potentially support- 

 ing structures projecting from their visceral arches, it was in- 

 herently extremely probable that these should be made use of 

 when actual supporting and motor appendages had to be developed 



1 Maurer also correlates the presence of the yolk sac with the absence of 

 external gills. He associates however the presence of the yolk sac not with the 

 disappearance of the external gills, but with their non-development, external gills 

 being according to him new formations, and homoplastic not homologous. 



2 Against their being new structures, secondarily developed as larval organs is 

 their general similarity in structure, connections, and position. Maurer thinks 

 they are secondary and would naturally develop where a large aortic trunk passed 

 close under the skin. I do not think so. The conditions favourable for develop- 

 ment of a dermal respiratory organ are afforded not by the presence of a large 

 vessel whose walls are thick and in any case are of relatively very small area in 

 proportion to cubic content, but rather of the small capillaries of the skin 

 generally, where the walls are small in thickness and of great area as compared 

 with the vessel's capacity. 



3 The question as to whether the external gills were not the main respiratory 

 organs of Vertebrates before the evolution of gill-slits, and whether the latter were 

 not developed as a means of renewing the water in contact with the external gills, 

 affords interesting subject for speculation. 



