§ 1. Life is lengthening - So far as we can judge from statistics of the average duration of life, it has been on the increase for three hundred and fifty years, and is now increasing more rapidly than ever before. During the seventeenth and eighteenth centuries the increase was at the rate of about four years per century; during the first three-quarters of the nineteenth century the rate was about nine years. At present in Massachusetts life is lengthening at the rate of about fourteen years per century; in Europe about seventeen; and in Prussia, the land of medical discovery and its application, twenty-seven. In India, where medical progress is practically unknown, the life span is short (twenty-five) and remains stationary.

§ 2. Table showing further practicable prolongation - It is possible to estimate the effect on the length of life of the partial elimination of various diseases. Using the statistics, experience, and estimate of 18 physicians as to the preventability of each of the list of 90 causes of death, we find that the length of life could easily be increased from forty-five to sixty, an increase of one-third, or fifteen years. This would result in a permanent reduction in death rate of about 25 per cent. The principal reductions would be from infantile diarrhea and enteritis, over 60 per cent of which could be prevented, with the result of an addition to the average length of life of 2.32 years. Broncho-pneumonia, also an infant disease, could be prevented to the extent of 50 per cent., whereby life would be lengthened by 0.60 year. Meningitis, which is usually fatal at the age of two, could be prevented by at least 70 per cent., and this prevention would lengthen the average life by 0.60 year. Eighty-five per cent of the mortality by typhoid fever is unnecessary, and if avoided would lengthen life at least 0.65 year. It would be feasible to prevent at least 75 per cent of cases of tuberculosis of the lungs, and thereby to lengthen life by about two years. If the deaths from violence were reduced only 35 per cent., human life would be increased by 0.86 year. The prevention of 45 per cent of cases of pneumonia would lengthen life by 0.94 year. These seven diseases alone could easily be reduced by these amounts so as to lengthen life by eight years. This could be done simply through insistence by the public on pure milk, pure water, pure air, and reasonable protection from accidents.

§ 3. Effect of prolongation at different ages. [Discussion of a diagram representing the life table of Massachusetts for 1893-1897.] It shows that about thirteen or more years could easily be added to the average duration of life. The diagram also shows the extent to which the additional life would fall in different ages. The per cent of life which would fall to the ages between 17 1/2 and 60, taken as the working period, would remain the same, namely, about 55 per cent.

§ 4. Fifteen years a minimum estimate - The estimate of fifteen years is a minimum because, first, it takes no account of future medical discoveries, such as a method of curing or preventing cancer and of postponing old age, as would second, it takes little account of the cumulative influence of hygiene. The full benefit of hygiene cannot be felt until it is practised throughout life, and not at the approach of specific danger. Most so-called "causes" of death are merely the last straws which break the camel's back. When a pure water supply prevents deaths from typhoid fever, it prevents two or three times as many deaths from other causes.