A prominent psychologist has maintained that reflexes are to be considered as the mechanical effects of acts of volition of past generations. The ganglion-cell seems the only place where such mechanical effects could be stored up. It has therefore been considered the most essential element of the reflex mechanism, the nerve-fibers being regarded, and probably correctly, merely as conductors. Both the authors who emphasize the purposefulness of the reflex act, and those who see in it only a physical process, have invariably looked upon the ganglion-cell as the principal bearer of the structures for the complex co-ordinated movements in reflex action. I should have been as little inclined as any other physiologist to doubt the correctness of this conception had not the establishment of the identity of the reactions of animals and plants to light proved the untenability of this view and at the same time offered a different conception of reflexes. The flight of the moth into the flame is a typical reflex process. The light stimulates the peripheral sense organs, the stimulus passes to the central nervous system, and from there to the muscles of the wings, and the moth is caused to fly into the flame. This reflex process agrees in every point with the heliotropic effects of light on plant organs. Since plants possess no nerves, this identity of animal with plant heliotropism can offer but one inference--these heliotropic effects must depend upon conditions which are common to both animals and plants. On the other hand, Watson, in his _Introduction to Comparative Psychology_, defines the reflex as "a unit of analysis of instinct," and this means that instinctive actions in man and in animals may be regarded as combinations of simple reflex actions, that is to say of "fairly definite and generally predictable but unlearned responses of lower and higher organisms to stimuli." Many of these reflex responses are not fixed, as they were formerly supposed to be, but "highly unstable and indefinite." This fact makes possible the formation of habits, by combination and fixation of these inherited responses.