TilNNEAN &OCIETY OF LONDON. 1 5 



remaineJ the same. Such a positive assertion, if true, immediately 

 puts out of court any theory which forms an ahmentary canal out 

 of something which is not hypoblast. It makes the alimentary 

 canal the keystone of the whole fabric of Evolution, not the central 

 nervous system. 



As I have pointed out in my book, the evidence of Brehm and 

 others is to the effect that there is no such morphological criterion 

 of hypoblast, but, on the contrary, the hypoblast is a physiological 

 conception rather than a morphological one, being the term given 

 to that layer which is found by its development to form the 

 digestive tube of the animal, and that in the earliest members of 

 the Metazoa, where we ought to expect the gastrula formation to 

 be most conspicuous, there it is most conspicuously absent, while 

 it is most clearly evident in those free-living pelagic blastula-forms 

 in which, owing to the absence of yolk, the necessity exists of 

 obtaining food from the outside even from the early blastula stage. 



According to the Law of Eecapitulation we may expect to find 

 in the developmental history of the Metazoa some indication of 

 the nature of the Protozoan ancestor which gave origin to the 

 Metazoa. Such indication is given with absolute uniformity in all 

 the Metazoa by the blastula stage, not by the so-called gastrula 

 stage. The blastula represents one of the highest Protozoan forms, 

 such, for example, as VoIvoa\ as I have suggested in my book, and 

 the blastula stage aifords yet another indication of the great law, 

 that the upward progress of the Animal Eace has always been 

 brought about by the genesis of the next highest form from a 

 member of the highest existing group of animals. 



Prof. E. W. MacBbidb, F.E.S. (Visitor), remarked :— 

 Dr. Gaskell has given us a brilliant exposition of his famous 

 theory of the " Origin of Vertebrates " to which it is impossible 

 to reply at all adequately in a quarter of an hour. Fourteen 

 years ago this theory was presented to the Cambridge Philo- 

 sophical Society and I then gave expression to many objections 

 which I felt to it ; and I confess that those objections remain in 

 unaltered force to-day. Not one of them has been removed by 

 Dr. Gaskell's speech, nor has a perusal of the latest edition of his 

 book weakened one of them in the slightest degree. 



The first and most fundamental objection is to the whole 

 nature of Dr. Gaskell's morphological reasoning. Unless this 

 kind of reasoning is to be guided by definite rules it becomes a 

 mere arena for the display of the imaginative faculties. The 

 change which one man regards as inconceivable another thinks the 

 most natural in the world. I, for instance, cannot contemplate 

 in cold blood a free-living animal giving up its alimentary canal 

 and beginning to digest with its skin, whilst to Dr. Gaskell this 

 seems the most natural transition in the world. But what rules 

 for morphological reasoning are suggested ? Tacitly or avowedly, 

 all zoologists agree on this — morphological reasoning must conform 

 to precedent. But what constitutes precedent in this case ? 



