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.

Mr. Wallace, in fact, appears to me to have mixed up two very distinct propositions:  the one, the indisputable truth that consciousness is correlated with molecular changes in the organ of consciousness; the other, that the nature of that correlation is known, or can be conceived, which is quite another matter.  Mr. Wallace, presumably, believes in that correlation of phenomena which we call cause and effect as firmly as I do.  But if he has ever been able to form the faintest notion how a cause gives rise to its effect, all I can say is that I envy him.  Take the simplest case imaginable—­suppose a ball