III. EXAMINATION OF MEASURES PROPOSED FOR DIMINISHING MORTALITY.--As the question of the population of France has been more especially discussed by the doctors, it has done great service as a vaulting board for medical theories. Doctors are very ready to reason as if they could dispose of human life at their will. It is very hard to keep a man from dying. The most skillful doctors have not reached that point; but it is very easy to have a man born, and is within the reach of the latest-made young practitioner. It is very doubtful whether the proposed measures will be efficacious or practical. See how much trouble we have had, after a century of experiments, in realizing the benefit of vaccination, the only nearly infallible remedy we have against disease. Surely a country ought to guard itself as much as possible against sickness and death, and should do everything that will conduce to that end, as we do all that is possible to cure a man ill with pneumonia or any other disease. But we should not delude ourselves with illusions, and we have to confess that the efficacy of the measures which we take to satisfy our conscience is very doubtful. The failures of hygiene are almost as numerous as those of medicine. Mortality has not increased in France. It is rather less there than in other countries in the same latitude, and even less than that of some of the countries situated farther north. So we can hardly hope to diminish it very much. The effect of mortality on the whole is, moreover, not to diminish natality, but rather to favor it. The death of an adult leaves some position vacant, and makes room for the institution of a new household and the birth of other children. So when a rich old man dies, the money he leaves helps set up his children in life; and when a poor old man dies, a burden is taken away from his descendants, who had to support him and who can now marry and have children. Some of the parallelisms in the movements of population which statisticians have observed may be explained thus. We might compare a human society to a tank so arranged as to be always full of water. It has a supply pipe (natality and immigration) which opens and operates only when the discharge pipe (mortality and emigration) is also open; or to a forest of definite extent, in which, when a clearing is opened, a new growth appears in the cleared space, unless some cause exists to prevent it, which cause it will be the forester's business to find and remove. He would not think, however, of stopping the cutting of the old trees, for that would be to prevent the essential condition of the new growth's getting a headway. The law of all living societies, in forests and in nations, is the perpetual renewal of the stock.