The manner in which the brain has formed itself in the evolution of the animal world will render the comprehension of its activities easier. Let us consider the simplest creatures, those possessing, so to speak, only a spinal cord. The nerves branching off from the upper part to the nostrils, eyes, ears, mouth, and elsewhere, were subjected during the long series of generations to more continuous stimuli than other nerves. The cells placed at the roots of these nerves were constantly excited by impressions from the external world; chemical processes and combustion would be more rapid in them, hence the necessity of a more copious flow of blood to those parts which were in greater activity. These cells multiplied rapidly at the roots of the organs of sense, gradually covering a wider field. As the animal structure became more perfect during evolution, and the relations of the animal to the outer world increased, the more abundant and active the cells at the roots of these nerves would become. We must not think here of one individual, although individual exercise does strengthen an organ, but must fix our eyes on the interminable chain of generations, all working in this direction. It was heredity (by which we still transmit to our children the structure and functions acquired by the nerve-centres) which, through the incessant efforts of our progenitors, enlarged this fertile field, until at last it resulted in the mass of the brain. If, on visiting a museum of comparative anatomy, the reader will look into the glass cases set apart for the study of the nervous system, he will see that the lowest animals have only a spinal cord, or a very small protuberance at the place corresponding to the brain. As the animal structure becomes more complicated, there is a visible increase of the protuberance, which enlarges gradually the nearer one approaches the superior animals, until at last it reaches its maximum size in man. II One of the greatest experimenters of modern physiology, Flourens, had already given it as his opinion that the whole cerebral mass performs the same functions in all its parts, and that if one portion be taken away, those contiguous to it charge themselves with its offices. This affords a partial explanation of the fact that wounds of the brain are far less dangerous than those of the spinal cord. It is always a great wonder, even to us physiologists, every time we convince ourselves on living subjects that the brain is without feeling. Men have been seen who suffered great portions of their brain, which protruded from the skull, to be cut away, and sick drunkards or madmen, who, through the wounds in their head, seized hold of the brain with their hands and destroyed it.