The mechanism by which these far-reaching changes in the instincts of animals are accomplished and transmitted by means of heredity to successive generations is one of the most obscure facts in medicine. The drunkard begets children predisposed to madness, just as the syphilitic transmit their curse to the innocent victims to whom they give life, but we know nothing of the manner of transmission; heredity of instinct remains inscrutable; the physiologist cannot yet confront such problems, so that he becomes a simple chronicler of the facts of which he does not know the laws, nor the intricate connecting threads. Brown-Séquard tried to subject this problem to experimental study, and obtained results which surprised all physiologists. He observed that guinea-pigs in which he had severed the sciatic nerve produced epileptic offspring, and that the destruction in male or female of certain parts of the nerve-centres caused marked malformation in the ears and eyes of the progeny. Pasteur found that the lambs of ewes that had been protected from a contagious malady called anthrax by inoculation with a diluted virus, were not attacked by this disease, and that even when inoculated with the active virus which would cause the death of other animals, they resisted it and did not succumb. This fact was confirmed by Toussaint and others. There were, indeed, many indications in science which led to the idea of protecting from diseases by means of heredity. If small-pox does not rage as formerly, if the victims are no longer so numerous, and if even the unvaccinated recover more easily, it is because a modification of our organism has been brought about through heredity and inoculation. Whenever this disease appears in a district which was never before infected, it rages as violently as formerly. The same thing takes place when the inhabitants of a country where this disease is unknown come to a town in the air of which the germs are present in abundance. The eight Eskimos who were brought a short time ago to the Jardin d’Acclimatation in Paris all died of the small-pox. It is a well-known fact that children of the same stock do not all resemble each other like stereotyped editions. Very often brothers and sisters, although they may have a striking physical resemblance to each other, show great difference of character; and what is of more importance to our study, these variations occur even though all the members of a family have been brought up in the same way.