252 JOURNAL OF COMPARATIVE NEUROLOGY. 
In the first place, of course, evolution is assumed. Whether 
this evolution has been catastrophic or uniform need not con- 
cern us here. It is sufficient to note that it is gradual, that is, 
a graded growth. 
In the second place, it is assumed that the psychical can- 
not have evolved from what we call the physical. To suppose 
that the psychical developed out of the physical is to contra- 
dict the scientific law of the conservation of energy. If the 
psychical grows out of the physical then examination should 
show that a certain amount of physical energy disappears as 
physical to reappear as psychical. But science does not find 
this to be true; the physical is not causally related to the psy- 
chical. Whatever the psychical may be, it falls outside physical 
energy, and science formulates this conclusion in its postulate 
of psycho-physical parallelism. Science does not attempt to 
penetrate into the puzzle of the relation of the psychical to the 
physical; it contents itself with the doctrine of parallelism as a 
working hypothesis. It finds the psychical as a sort of white 
elephant on its hands. It cannot deny it and thus get rid of it, 
Nor can it accept it and incorporate it into itself. Hence science 
simply says that the physicial and the psychical stand side by 
side, and lets the problem rest there, or hands it over to 
metaphysics. 
But we cannot escape the difficulty so easily here, since 
our subject involves just this question of the nature of the psy- 
chical. We may set aside materialism and spiritualistic ideal- 
ism and the theory of interactivity, all of which hold to the 
view that the physical and the psychical are causally related, 
and confine our attention to this theory of parallelism which 
professes at least not to contradict any known scientific law. 
Parallelism is the current doctrine, not only of modern science, 
but of much modern philosophy. There are two types. One 
is complete parallelism, holding that the physical and the psy- 
chical run parallel throughout. The other is partial, holding 
that the psychical is parallel to the physical so far as the psychi- 
cal extends, but that it forms a shorter series than the physical. 
According to the first theory every psychosis has its neurosis 
