1896.] The Biologie Origin of Mental Variety: 971 
kinds of artificial stimulation, the sensation resulting thereby 
is always the same for each nerve. For example, the cut stump 
of the optic nerve will respond to pinching, pricking, burning, 
and to chemical and electric stimulation; but always with an 
indefinite visual flash, whatever the form of stimulus that is 
applied. 
The chief facts upon which those who agree with Prof. 
Wundt base their opinion, are summed up within certain 
alleged phenomena of “Substitution,” there being some reason 
to believe that when parts of the cortex are destroyed, either 
by disease or by experimentation upon animals, certain re- 
maining parts take upon themselves the former functions of 
the lost parts, and change their former habits and modes of 
response in so doing. 
Neither of these opposing theories are conclusively substan- 
tiated at present, since there are counter replies for each. 
Thus it is open for Prof. Wundt to explain the fact of the optic 
nerve replying invariably with sight sensations to every sort of 
artificial stimulation, by saying that this is only true in the 
adult, where the cells, by having only one form of stimulation 
brought to them by the nerve to which they are permanently 
fixed, have been educated persistently in one form of response 
past the age when they have lost the power of plasticity and 
of shaking off the old habit to take on a new one—a power 
which at birth they eminently possessed. And, on the other 
side, it is open for those who believe in fixed congenital re- 
sponses to suspect that all the facts of Substitution are due to 
the lost function being taken up by remaining cells of the same 
kind, and especially by the correspondent cells of the other 
half of the body; i. e., those of like kind in the opposite lobe 
of the brain. 
= Such is the state of this controversy up to date ; and its con- 
fusion would be out of place in our present study if we were 
not now able both to bring this problem, by new considerations, 
to a solution, and also to demonstrate its cogent bearings upon 
our main subject. We speedily come to this by recalling that 
we have already determined that the molecular differences, cor- 
responding to the differences between our several senses, are 
