Professor Hering and I, to use a metaphor of his own, are as men who have observed the action of living beings upon the stage of the world, he from the point of view at once of a spectator and of one who has free access to much of what goes on behind the scenes, I from that of a spectator only, with none but the vaguest notion of the actual manner in which the stage machinery is worked. If two men so placed, after years of reflection, arrive independently of one another at an identical conclusion as regards the manner in which this machinery must have been invented and perfected, it is natural that each should take a deep interest in the arguments of the other, and be anxious to put them forward with the utmost possible prominence. It seems to me that the theory which Professor Hering and I are supporting in common, is one the importance of which is hardly inferior to that of the theory of evolution itself—for it puts the backbone, as it were, into the theory of evolution. I shall therefore make no apology for laying my translation of Professor Hering’s work before my reader. Concerning the identity of the main idea put forward in “Life and Habit” with that of Professor Hering’s lecture, there can hardly, I think, be two opinions. We both of us maintain that we grow our limbs as we do, and possess the instincts we possess, because we remember having grown our limbs in this way, and having had these instincts in past generations when we were in the persons of our forefathers—each individual life adding a small (but so small, in any one lifetime, as to be hardly appreciable) amount of new experience to the general store of memory; that we have thus got into certain habits which we can now rarely break; and that we do much of what we do unconsciously on the same principle as that (whatever it is) on which we do all other habitual actions, with the greater ease and unconsciousness the more often we repeat them. Not only is the main idea the same, but I was surprised to find how often Professor Hering and I had taken the same illustrations with which to point our meaning. Nevertheless, we have each of us left undealt with some points which the other has treated of. Professor Hering, for example, goes into the question of what memory is, and this I did not venture to do. I confined myself to saying that whatever memory was, heredity was also. Professor Hering adds that memory is due to vibrations of the molecules of the nerve fibres, which under certain circumstances recur, and bring about a corresponding recurrence of visible action. This approaches closely to the theory concerning the physics of memory which has been most generally adopted since the time of Bonnet, who wrote as follows:—