THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 33 
vertebrate, but also to a distinct ancient type of arthropod, the 
Palzostracan, 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 
