EVOLUTION OF THE VERTEBRATE 191 



brate type this change in the position of the cord is clearly 

 a very puzzling matter. 



The theory of Gaskell is at the present time generally 

 held to explain satisfactorily how the invertebrate ventral 

 cord came to take a dorsal position, being roughly to the 

 effect that in some primitive invertebrate type, possessing 

 annelid characters, the nerve-ganglia composing the ventral 

 chain increased in size and grew upwards so as to surround 

 or encase the overlying alimentary canal, the lumen of this 

 finally becoming the ventricles of the brain and the central 

 canal of the cord. Also that in place of the original digestive 

 canal a new one was formed underneath the cord. 



We cannot pretend to find this theory in any way 

 satisfactory, and it is here suggested that there lies at hand 

 a much simpler one, and one completely in harmony with 

 the theory of the evolution of Continuity. 



There is more reason to believe that vertebrates have 

 not evolved from any annelid type, or one possessing a 

 ventral chain and oesophageal collar ; and that the primitive 

 Fish evolved on a line of its own from a special continuously 

 medusoid type, while the annelids did the same from 

 another. Our firm belief is that the Vertebrates sprang 

 suddenly into existence with the primitive Fish, and that the 

 spinal cord was dorsal from the start. That an inherited 

 alimentary canal should come to take on the duties of a 

 spinal central canal, as Gaskell's theory suggests, is, to say 

 the least, improbable. It would be a sudden modification 

 of a kind which completely ignored the inheritance of func- 

 tion and plan. Did we entertain the suggestion as possible, 

 it could only be by picturing it as occurring during develop- 

 ment ; but we would ask how a new alimentary tract could 

 be formed from developing tissues whose potentialities had 

 been decided in simpler living types countless ages before. 



It is true that we can recognise the evolution of the 

 closed vascular system of the vertebrate from the hollow 

 tubular interiors of ancestral sporosacs, but this is a different 

 question altogether, for it is a harmonious evolution by 

 steps which are the repeated multiplications of Continuity. 

 We can see how the digestive cavity of the zooid was multi- 

 plied in the medusoid into a digestive cavity with thin radial 

 branches, whose contents bearing oxygen and nourishment, 



