CRANIAL NERVES AND SENSE ORGANS OF FISHES. 235 



less easy prophecy to foretell that he who discovers the 

 phylogeny of the sense organs will have solved the more 

 important problems involved in the evolution of the 

 vertebrate head. The visceral system and its nerves 

 present independent difficulties, which the study of the 

 branchial nerves has hitherto done little to dispel. 



The old view that the head was a modified part of 

 a uniform body naturally produced many attempts to 

 homologise the cranial with the spinal nerves. The 

 necessity of reducing the cranial nerves to a number of 

 metameric structures, each having representatives of dorsal 

 and ventral roots, resulted in the publication of many 

 theories, often mutually destructive, but some of great 

 ingenuity. The whole discussion completely begged the 

 fundamental question that the cranial nerves were 

 modified spinal nerves, and the view that both might 

 have totally different phylogenetic histories was hardly 

 contemplated or provided for. We now find that these 

 attempts are ceasing, and that morphologists are begin- 

 ning to realise that the cranial nerves cannot be serially 

 compared with the spinal nerves. The apparent conclu- 

 siveness which formerly characterised their efforts is 

 another illustration of how easy it is to compare one 

 structure with another, and how often it transpires that 

 such comparison is absolutely without value. The 

 essential difference that exists then between cranial and 

 spinal nerves, and the futility of all efforts to show that 

 they are essentially serial in character, may and has been 

 explained as due to either of two causes. One is that 

 the head is phylogenetically the older portion of the 

 vertebrate organism, and the other the opposite view 

 that it represents the younger portion. In either case, 

 the absolute distinction that exists between cranial and 

 spinal nerves is, of course, explained ; but, to my mind, 



