964 The American Naturalist. [December, 
some of which ring bells while others blow whistles. For if 
it be asked why our sensory currents ‘ring up’ such different 
results as sight from the optic nerve, and hearing from the 
auditory nerve, it is plainly not satisfactory to answer, “ be- 
cause we have eyes and ears,” if, as this doctrine asserts, the 
eye and ear nerve currents are alike. Nor is it much more 
enlightening to be told that “it is the place in the cortex to 
which the different nerves run that makes the difference in 
the sensation resulting from them;” not unless we are in 
some way told wherein and why these “ places” differ. It is 
just in the fact of never having even inquired how these 
“places” came to differ, that our evolutionary science falls 
short in one of the most curiously interesting and important 
questions that can arise either in biology or in psychology. 
Of course, it is a fundamental assumption of both these 
sciences that all our mental differences are paralleled by mole- 
cular differences among the neural activities that underlie 
them. But this still avoids the question why these last are 
different, and how they came to beso. And until some an- 
swer shall be found that shall logically connect these ultimate 
neural peculiarities with those peculiarities in outer objects 
which the world commonly conceives to correspond to our 
various sights and sounds, it can scarcely be boasted that we 
are much less naive than the ancients who thought that the 
objects gave off films that floated into our minds bodily. I 
by no means imply that this doctrine of all sensory nerve cur- 
rents being of the same sort is universally accepted. But 
where any other hypothesis has been offered in its place, the 
relationship between inner sense and outer stimulus has been 
left as barren of explanation as even in this doctrine, where, 
apparently, the possibility of explanation is cut off altogether. 
But all these matters we are to examine categorically further 
on. Sufficient has now been said, by way of introduction, to 
make clear that it is the variety of our sensory responses (with- 
out which our minds would not be minds), and of their con- 
nection with the sorts of stimuli with which they are now 
connected, that we are, in this paper, to subject to careful in- 
vestigation. It should be obvious that this inquiry must in- 
