THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 55 
together in the caudal region to form what is known as the caudal 
brain. 
The brain in these animals may be spoken of as composed of 
three parts—(1) the fused supra-cesophageal ganglia, (2) the fused 
prosomatic ganglia, and (3) the fused mesosomatic ganglia, Such a 
brain is strictly homologous with the vertebrate brain, which also is 
built up of three parts—(1) the part in front of the notochord, the 
prechordal or supra-infundibular brain, which consists of the cerebral 
hemispheres, together with the basal and optic ganglia and corre- 
sponds, therefore, to the supra-cesophageal mass, with its olfactory 
and optic divisions lying in front of the esophagus; (2 and 3) the 
epichordal brain, composed of (2) a trigeminal and (3) a vagus divi- 
sion, of which the first corresponds strictly to the fused prosomatic 
ganglia, and the second to the fused mesosomatic ganglia. Further, 
just as in the embryo of an arthropod it is possible, with more or 
less accuracy, to see the number of neuromeres or original ganglia 
which have fused to form the supra- and infra-cesophageal portions 
of its brain, so also in the embryo of a vertebrate we are able at 
an early stage to gain an indication, more or less accurate, of the 
number of neuromeres which have built up the vertebrate brain. 
The further consideration of these neuromeres, and the evidence they 
afford as to the number of the prosomatic and mesosomatic ganglia 
which have formed the epichordal part of the vertebrate brain, must 
be left to the chapter on the segmentation of the cranial nerves. 
The further continuation of this process of concentration of 
separate segments, together with the fusion of the nervous system 
with the tube of the alimentary canal, leads in the simplest manner 
to the formation of the spinal cord of the vertebrate from the meta- 
somatic ganglia of the ventral chain of the arthropod. 
Tort ANTAGONISM BETWEEN CEPHALIZATION AND ALIMENTATION. 
This concentration of the nervous system in the head-region, 
together with an actual increase in the bulk of the cephalic nervous 
masses, constitutes the great principle upon which the law of upward 
progress or evolution in the animal kingdom is based, and it illus- 
trates in a striking manner the blind way in which natural selection 
works; for, as already explained, the central nervous system arose as 
a ring round the mouth, in consequence of which, with the progressive 
