On the evidence of individual cases the innocuousness of many poisons and diseases might be proved. Individuals are sometimes found to resist inoculation. It is a singular, and as yet unexplained fact, that centenarians are often found in the greatest proportion in times and places where the average duration of life of the whole population is very low. It has been shown from an accurate registration of centuries in Geneva, that as the average duration of life amongst the whole community advanced, the proportion of extreme cases of centenarians diminished. According to the bills of mortality there were nearly three times as many centenarians in London a century ago than at present. Out of 141,720 deaths within the bills of mortality during the five years ended 1742, the deaths of 58 persons alone of 100 years and upwards of age are recorded; whilst out of 139,876 deaths which occurred in the metropolis as returned by the registrar-general, during the three years which ended 30th June, 1841, only 22 deaths of 100 years of age and upwards are recorded. The average age of death of all who died was then 24 years; it is now, judging from an enumeration made from the returns of 1839, about 27 years; and there appears to have been a considerable improvement in all periods of life up to 90 years. Footnote 6: _Vide_ Appendix of the district returns of the Mortuary Registration. Footnote 7: In the medical profession examples are not rare of the attainment of extreme old age; yet as a class they bear the visible marks of health below the average. The registration of one year may be an imperfect index; but the mortuary registration for the year 1839 having been examined, to ascertain what was the average age of death of persons of the three professions, it appears that the average age of the clergymen who died in London during that year was 59, of the legal profession 50, and of the medical profession 45. Only one medical student was included in the registration: had the deaths of those who died in their noviciate been included, the average age of death of the medical profession would have been much lower. Footnote 8: An instance in exception of a barber having caught fever is subsequently stated. Footnote 9: