Hence the physical constitution of the individual is that which has primary importance. That may be considered first as an individual question, without going beyond the circumstances of the personal life and health, a purely _somatic_ investigation. We may next inquire how many of his peculiarities the individual owes to his ancestors, which will bring up the questions of heredity, hybridity, and others, including mental as well as physical traits. His debt is large to these, but still larger, say some writers, to his contemporaries, the associates with whom he has been thrown from birth. These are his “people,” the “group” of which he is a member. He is modified in a thousand ways by this “demographic” environment. All these—his ancestors, fellows, and his own body—are “human” influences. Beyond them lies the great world of other beings and of unconscious forces, the animals and plants, the land and water, the clime and spot, which make up his “geographic” environment. How dependent is he upon these! How utterly they often control his thoughts and actions! Each of these I shall endeavour to estimate in their influence on the individual, not as an individual, but as a member of a group; and on the group itself, as an independent, psychic entity, nowise identical in character with any individual. CHAPTER I _THE INFLUENCE OF THE SOMATIC ENVIRONMENT_ The human body is an “organism” each part of which is in vital relation to the whole, and is influenced by the condition of every other part. This is true of function as well as structure, for function, after all, is merely the term we give to structure in action. Mentality, psychical activity, is a function, and, like all others, is organically conditioned by the whole organism and its several parts. To understand the influence of the body on the mind, therefore, we should consider in such relation each of the physiological “systems” which make up the organic life. For my present purpose, however, it will be sufficient to select those most closely related to mental activity. _The Brain._—The learned of all times have sought to find “the seat of the soul.” Primitive men generally placed it in the liver or in the heart; but anatomists have been long agreed that it must be somewhere in the head. The latest word from them is that it resides in the nerve cells of the grey matter of the brain, in the number and activity of the “pyramid-neurons” there situate, and probably in their capacity to send out shoots or branches. This intimate, ultimate, structure and potency establishes the difference between the intellectual faculties of species and individuals. In the lower animals these cells are few and scattered, and their proliferations short and simple. In man the cells increase in number and their extensions become long and complex. They are more abundant when the grey matter is ample, as is the case where the convolutions are intricate.