But why do we blush? some will ask, who insist on penetrating to the root of things. Why, under certain conditions, does the blood flow more abundantly into the rabbit’s ear and the human face? The answer to this question will be better understood when I have shown that the brain also becomes redder after an emotion. For the maintenance of life it is necessary that a dilatation of the blood-vessels should take place in all those organs in which a disturbance occurs. We all know that when our hand has been firmly squeezed, or when we have received a blow or contusion, the skin reddens at once. This change in the circulation is indispensable, for the more copious flow of blood to that part which has suffered an arrest of nutrition serves to renew the vital processes and to repair the damage caused by the injury. The same phenomena appear in the brain under psychic conditions. Emotion occasions greater energy in the chemical processes of the brain; there is a modification in the nutrition of the cells, the nervous force is more rapidly consumed, and therefore the expansion of the blood-vessels of head and brain tend, by a more abundant supply of blood, to preserve the activity of the nerve-centres. It is in the tissues, in the properties of the living substances which constitute the vital machine, that we must seek the reasons of numerous phenomena which Darwin deduced from external causes, natural selection or environment. We shall endeavour to confine within much narrower limits the effects of chance, will, and accident, which play such an important part in Darwin’s theory. Nothing is the result of a creative force serving a premeditated end; organisms have formed and changed themselves through causes exclusively mechanical. Work perfects organisms, and the operative parts undergo, through their own activity, far-reaching modifications, which render their structure still more perfect. CHAPTER I _HOW THE BRAIN WORKS_ I Before beginning the study of the nerve-centres I shall remind the reader of a few very simple facts, which, doubtless, he already knows, but which, recalled, will render more apparent the part taken by the body in the functions of the mind. In order to know how the brain works, it is sufficient to recall the pictures and visions which pass before us when we are absent-minded. How curious it is when the mind sets out on its fanciful wanderings! when, unconsciously, we leave the everyday world behind us and stand motionless, with open eyes, seeing and hearing nothing. How often in the quiet of our study, while reading a book, have we not seen the words gradually fade one into another, until we found ourselves as though enveloped in a cloud, far away amid the recollections of childhood or the hopes of the future! And what wonderful forms grow out of the flames, the logs, and the sparks glowing under the ashes, when we draw close to the fire in lonely evenings!