THE ANCESTRY OF THE VERTEBRATES 85 



of the central nervous system in the Annelid and 

 in the Vertebrate are exactly and diametrically re- 

 versed. In the Annelid the brain is dorsal to the 

 alimentary canal while the nerve-cord is ventral. In 

 the Vertebrate if we regard the brain as dorsal, then 

 the central nervous cord runs along the back and is 

 also dorsal to the alimentary canal. A diagram will 

 make this clear (see Fig. 19 A, B and c). 



It is possible to overcome this difficulty, but the 

 process of overcoming it involves an hypothesis which 

 to many appears to overstep the bounds of temerity. 

 In order really to homologize the Annelid part by 

 part with the Vertebrate, we must suppose that the 

 dorsum of the Vertebrate corresponds to the ventrum 

 of the Annelid, that the brain of the Vertebrate does 

 not correspond to the brain of the Annelid but to its 

 suboesophageal ganglion, and that the mouth of the 

 Annelid has migrated in the Vertebrate from its 

 original ventral to a terminal and finally a dorsal 

 position. This is made clear in the diagrams pre- 

 sented in Fig. 19. 



Although it must be admitted that the hypothesis 

 is a bold one, it cannot be charged with any inherent 

 absurdity from a morphological standpoint. It may 

 seem a revolutionary, almost impious, idea that what 

 we have been accustomed to look upon as the back 

 and the front of ourselves and other Vertebrates, 

 have all along been diametrically the opposite ; that 



