THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 33 



vertebrate, but also to a distinct ancient type of arthropod, the 

 Palseostracan, the only living example of which is the King- Crab or 

 Limulus ; while the nearest approach to the trilobite group among 

 living arthropods are Branchipus and Apus. It follows, therefore, that 

 for the following up of this clue, Limulus especially must be taken into 

 consideration, while Branchipus and Apus are always to be kept in mind. 



Ammoccetes rather than Amphioxus is the Best Subject for 



Investigation. 



It is not, however, Limulus that must be investigated in the first 

 instance, but the vertebrate itself; for it can never be insisted on too 

 often that in the vertebrate itself its past history will be found, but 

 that Limulus cannot reveal the future of its race. What vertebrate 

 must be chosen for investigation ? Reasons have been given why 

 oUr attention should be fixed upon the king-crab rather than on the 

 lobster on the invertebrate side ; what is the most likely animal on 

 the vertebrate side ? 



From the evidence already given it is manifest that the earliest 

 mammal belonged to the lowest group of mammals ; that the birds 

 on their first appearance presented reptilian characteristics, that the 

 earliest reptiles belonged to a low type of reptile, that the amphibians 

 at their first appearance were nearer in type to the fishes than were 

 the later forms. As each of these groups advances in number and 

 power, specialization takes place in it, and the latest developed 

 members become further and further removed in type from the 

 earliest. So also it must have been with the origin of fishes : here 

 too, in the quest for information as to the structure and nature of 

 the first-formed fishes, we must look to the lowest rather than to 

 the highest living members of the group. 



The lowest fish-like animal at present living is Amphioxus, and 

 on this ground it is argued that the original vertebrate must have 

 approached in organization to that of Amphioxus ; it is upon the 

 comparison between the structure of Amphioxus and that of Balano- 

 glossus, that the theory of the origin of vertebrates from forms like 

 the latter animal is based. For my own part, I think that in the 

 first instance, at all events, Amphioxus should be put on one side, 

 although of course its structure must always be kept in mind, for 



the following reasons 



D 



