LIXXEAN SOCIETY OF LO^fDOX. 37 



It appears from Dr. Gaskell's opening speech that he assumes 

 that the anterior opening of the neural tube in the larval Amphi- 

 oxiis represents the old Arthropod mouth, but in the higher 

 A'^ertebrates he locates this ancestral mouth in the region of the 

 infundibulum. This necessitates the supposition that the anterior 

 neuropore is identical in position with the infundibulum, a 

 supposition wliiuh would, I imagine, strike modern embryologists 

 with amazement. 



Then again, what is the value of the evidence afforded by the 

 so-called neurenteric canal? This structure, if structure it can 

 be called, simply results from the fortuitous enclosure of the 

 blastopore by the uprising neural folds, and to my mind it 

 has no phvlogenetic significance of the kind attributed to it by 

 Dr. Gaskeil. 



It was urged, I think by Professor Starling, that the immense 

 physiological importance of the central nervous system gives it a 

 special claim to consideration as evidence in the discussion of the- 

 origin of Vertebrates. This is entirely contrary to the usually 

 accepted views of systematic zoologists, who find in structures 

 which are apparently of the least use to their possessors * the 

 best guides to genetic affinity. Organs which are of great use 

 must be subject to adaptive modification in accordance with the 

 changing needs of the organism. Modern schemes of classification 

 are indeed largely based upon this principle, and certain modi- 

 fications in the nervous system of tape-worms have been explicitly 

 ruled out as guides to classification in accordance therewith. 



[The central nervous system of a Vertebrate of course agrees 

 with that of an Arthropod in exhibiting traces of a fundamental 

 metamerism, because both Vertebrates and Arthropods are meta- 

 merically segmented animals, and both have very probably been 

 derived from some metamerically segmented common ancestor. 



It is the later modifications, coenogenetic rather than palingenetic 

 features, readily explicable as adaptations to the special needs 

 of the Vertebrate organisation (which are of course in many 

 respects similar to those of the Arthropod organisation), that I 

 consider to be inadmissible as evidence in considering the phylo- 

 genetic relationships of the Vertebrates. The fact that highly 

 specialized characters of the brain may afford a useful clue to 

 relationship within the limits of the Vertebrate phylum does not, in 

 my opinion, affect the question at issue. In dealing with closely 

 related groups comparatively recent modifications are oi:' undoubted 

 taxonomic value; but in comparing such widely divergent groups 

 as Vertebrates and Arthropods, resemblances due to such characters, 

 when they can be explained quite reasonably as the result of con- 

 vergent evolution, must be eliminated from the discussion.] 



* I may cite in illustration the microsoleres or so-called flesh-spicules of 

 siliceous sponges, wiiii their cxtraDi-diuarily diverse and rtp|)arently specifically 

 constant niodilications. 



