THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 9 



thus zoologists are able to speak definitely of the Vertebrata, Arthro- 

 poda, Annelida, Echinodermata, Porifera, Ccelenterata, Mollusca, 

 etc. In each of these groups affinities can be traced between the 

 members, so that it is possible to speak of the progress from lower 

 to higher members of the group, and it is conceivable, given time to 

 work out the details, that the natural relationships between the 

 members of the whole group will ultimately be discovered. 



Thus no one can doubt that a sequence of the kind has taken 

 place in the Vertebrata as we trace the progress from the lowest fishes 

 to man, and already the discoveries of palaeontology and anatomy 

 give us a distinct clue to the sequence from fish to amphibian, from 

 amphibian to reptile, from reptile to mammal on the one hand, and 

 to bird on the other. That the different members of the vertebrate 

 group are related to each other in orderly sequence is no longer a 

 matter of doubt ; the connected problems are matters of detail, the 

 solution of which is certain sooner or later. The same may be said 

 of the members of any of the other great natural groups, such as 

 the Arthropoda, the Annelida, the Echinodermata, etc. 



It is different, however, when an attempt is made to connect 

 two of the main divisions themselves. It is true enough that there 

 is every reason to believe that the arthropod group has been evolved 

 from the segmented annelid, and so the whole of the segmented 

 invertebrates may be looked on as forming one big division, the 

 Appendiculata, all the members of which will some day be arranged 

 in orderly sequence, but the same feeling of certainty does not exist 

 in other cases. 



In the very case of the origin of the Appendiculata we are con- 

 fronted with one of the large problems of evolution — the origin of 

 segmented from non-segmented animals — the solution of which is not 

 yet known. 



Theories of the Origin of Vertebrates. 



The other large problem, perhaps the most important of all, is the 

 question of the relationship of the great kingdom of the Vertebrata : 

 from what invertebrate group did the vertebrate arise ? 



The great difficulty which presents itself in attempting a solution 

 of this question is not so much, as used to be thought, the difficulty 

 of deriving a group of animals possessing an internal bony and 



