=All Mental Process Accompanied by Neural Process.--=Whatever the ultimate decision of psychologists may be regarding the relation of mind to the sensory and nervous mechanism of man it is certain that there is so close an association between them that the least alteration in the mechanism means a parallel effect in the mind, or in the words of Huxley, "every psychosis is definitely correlated with a neurosis." The rind or _cortex_ of gray matter which constitutes the surface of the large cerebral hemispheres of the human brain is regarded as the seat of consciousness. The development of the mental powers in the infant is dependent on the development of the elements of this cortical substance and the waning of the mental faculties in old age goes hand in hand with its atrophy. Abnormal arrangements, injuries or omissions in it mean mental unsoundness. How the activity of the structural mechanism gives a reaction in consciousness is not understood, but we know that in the living being the two phenomena are inseparably linked. Whether we accept the hypothesis that consciousness is an actual product of the structural mechanism or the hypothesis that the latter is only an instrument for the manifestations as consciousness of an outside force or entity, just as the telegraphic instrument manifests the existence of electricity, is neither here nor there for our purposes. On either supposition the degree and manner of expression are determined by the structure of the mechanism. Our main problem is to decide as nearly as possible how much of the mechanism is rigidly inherited, how much is at birth largely undestined, so that its ultimate outcome is in part a product of the forces which play upon it, or in other words of education and training. =Gradation in Nervous Response from Lower Organisms to Man.--=To comprehend fully the basic nature of human neural responses one must seek the roots in the behavior of lower organisms. For there is found in a simpler form many of the fundamental activities and the first dim gropings which emerge in man as memory, reason and will. As we ascend the scale of animal life we find a continuous advance in neural complexity and nervous response that in many respects grades up closely to the human type. A windmill or a weather-vane points toward the source of the wind, obviously not because either exercises any special choice in the matter, but because it is constructed on such lines of symmetry that when the wind strikes it, if it slants the slightest to left or right, the more exposed surface receives the greatest pressure and thus swings the body back into the line of least resistance.