38 Dr Gashell, The Origin of Vertebrates. [Nov. 25, 



If such assumptions were allowed the descent of any animal 

 from any other could be proved. 



Mr S. F. Harmer pointed out that Dr Gaskell had based his 

 conclusions on two main lines of argument; the first being the 

 assumption that the comparative study of any particular organ 

 within the limits of a single group will not only enable us to 

 retrace the steps which evolution has taken, but will give some 

 clue to the conditions which existed prior to the evolution of that 

 organ as such ; and the second being derived from a detailed 

 comparison of Limulus with Vertebrates. 



Instances were given to show that the first assumption is 

 fallacious. The evolution of the lung may have been retraced to 

 the simplest form in which that organ exists in Vertebrates, but 

 it does not follow that the line can be produced backwards so as 

 to give any indication of the conditions antecedent to the evolu- 

 tion of the air-bladder. Nor does the retracing of the line of 

 evolution of the Perissodactyle hand or foot give any information 

 with regard to what preceded the pentadactyle condition. 



It was admitted that the resemblances between Limulus and 

 Vertebrates were, on Dr Gaskell's showing, of a remarkable nature ; 

 but many of the details could be satisfactorily criticized only by 

 one who has made a special study of both Limulus and the 

 An'imoccet. It was, however, suggested that since the resem- 

 blances extended to such minute points of detail, it would be 

 remarkable, if Dr Gaskell's theory were true, that differences 

 should exist in what would appear to be fundamental points ; 

 as, for instance, the numerical difference between the branchial 

 appendages of Limulus and the branchial bars of Petromyzon. 



Dr Gaskell concludes that the fact that the intestine is elon- 

 gated in the higher Vertebrata and straight in the Ammocoet 

 points to an original " approximation of the cranial and sacral 

 portions of the gut " ; and that therefore the branchial chamber 

 formerly terminated close to the cloaca. It would equally well 

 follow from this line of argument that other groups of animals in 

 which the intestine of the higher members is convoluted and of 

 the lower forms straight were also descended from ancestors in 

 which a secondary alimentary canal had been evolved. 



The argument from the position of the yolk in Vertebrate 

 embryos has no real weight. Dr Gaskell, in dealing with the 

 Vertebrata, has put a special interpretation on a very general 

 embryological phenomenon. He has entirely ignored other groups 

 of animals, in this respect; and he is no more justified in his 

 conclusion that the embryological evidence indicates a late evolution 

 of the mid-gut in Vertebrates than he would have been in drawing 

 a similar inference from the developmental history of Mollusca. 



On the motion of Mr H. Gadow the discussion was adjourned. 



