revity as is compatible with clearness,
that this theory is not only incapable of proof, but is also,
as it appears to me, inconsistent with accurate conceptions of
molecular physics."

With all respect for Mr. Wallace, it appears to me that his remarks
are entirely beside the question. I really know nothing whatever, and
never hope to know anything, of the steps by which the passage from
molecular movement to states of consciousness is effected; and I
entirely agree with the sense of the passage which he quotes from
Professor Tyndall, apparently imagining that it is in opposition to
the view I hold.

All that I have to say is, that, in my belief, consciousness and
molecular action are capable of being expressed by one another, just
as heat and mechanical action are capable of being expressed in terms
of one another. Whether we shall ever be able to express consciousness
in foot-pounds, or not, is more than I will venture to say; but
that there is evidence of the existence of some correlation between
mechanical motion and consciousness, is as plain as anything can be.
Suppose the poles of an electric battery to be connected by a platinum
wire. A certain intensity of the current gives rise in the mind of a
bystander to that state of consciousness we call a "dull red light"--a
little greater intensity to another which we call a "bright red
light;" increase the intensity, and the light becomes white; and,
finally, it dazzles, and a new state of consciousness arises, which we
term pain. Given the same wire and the same nervous apparatus, and the
amount of electric force required to give rise to these several states
of consciousness will be the same, however often the experiment
is repeated. And as the electric force, the light-waves, and the
nerve-vibrations caused by the impact of the light-waves on the
retina, are all expressions of the molecular changes which are taking
place in the elements of the battery; so consciousness is, in the same
sense, an expression of the molecular changes which take place in that
nervous matter, which is the organ of consciousness.

And, since this, and any number of similar examples that may be
required, prove that one form of consciousness, at any rate, is, in
the strictest sense, the expression of molecular change, it really
is not worth while to pursue the inquiry, whether a fact so easily
established is consistent with any particular system of molecular
physics or not.