THE EVIDENCE OF 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 paleontology 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, ete. 
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 UF 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 
