It is with heredity as with certain chemical combinations arranged in kindred categories because of similarity of structure and composition, although one is noxious, the other beneficial, one poisonous, the other neutral. Even in twins joined together--there are several cases in the annals of medicine--in those also which I studied together with Professor Fubini, who are connected at the lower part of the trunk and have only two legs together, and who must certainly have always lived under the same conditions, there are yet profound differences of character. We must therefore distinguish between the hereditary and the personal character, the characteristics of the family and those of the individual. IV The greater the advance of science, the greater should be the authority of the physician in education. All pedagogic systems deviating from natural means lead us into error and into morbid conditions of mind and body. Education should be conducted according to the laws of life, the needs of the organism, and the material interests of society. The study of all that relates to the development of the intellectual faculties, the cure of aberrations of instinct and moral defects caused by the turbulence of the passions are problems so closely connected with phenomena of the physical order, that the physiologist and the physician should devote their attention to them as to a biological fact, as to the cure of a disease. Unhappily, even considered from this point, the problem of education presents most serious difficulties. Some passions are incurable; others the body cannot resist, but wastes rapidly away under them, as under the fatal sway of a galloping consumption. The will does not suffice, for itself is only the result of the vitality of the organism, and of the greater or lesser resistance of which the nervous system feels itself capable. The succession of causes and effects often forms an indissoluble circle which man cannot break with the simple force of his will. _Weakness produces fear, and fear produces weakness._ Here is a fatal revolution in the functions of the organism. Of what use are the arbitrary and imaginary distinctions philosophers have made in the functions of the mind, when they cannot be separated from those of the body? There are in life fatal cliffs, currents which we cannot stem, and which carry us to inevitable destruction. _Weakness increases excitability, excitability foments lasciviousness, and lasciviousness in its turn begets weakness._ Here the functions of the organism are like a gaping whirlpool, like an avalanche moving onwards and dragging us to the fatal precipice, does a foot but slip on the path of life.