The determining factors of long life may be broadly divided into those included under heredity, environment, due functional activity, and personal habits. It is impossible to separate these factors into watertight compartments, for a certain amount of overlapping between them is unavoidable. HEREDITY The influence of heredity has often been insisted upon and is perhaps the most important factor in longevity. Out of 824 persons between 80 and 100 years of age analysed by Humphry, 406, or 49·4 per cent, came of long-lived families. Numerous striking examples of such families are on record, but a few only need be given. Roy[45] quotes the case of Dr. Iverex, who died in 1700 at the age of 104, his father at 112, his mother 107, and his grandfather 130 years, and gives four other examples of three centenarians in the same family. Another remarkable family group was that of Joseph Retas who died at Tarbes at the age of 118 in 1888, and was the son of a man aged 111 years and had a brother aged 114 years. Two centenarian twin sisters were recorded in a village near Athlone[46]; centenarian sisters and brothers are not so very exceptional. Sir Hermann Weber[47] recorded two families, in one of which the average age of ten children was over 90 years and in the other of eight children nearly 90 years. A good stock may ensure long life in the face of adverse environment, such as town life and alcoholism; thus Dr. John Brownlee[48] showed statistically that while persons dying at the age of 51 in the average environment would have, had they lived in the country, a mean life of seven years longer, this difference was less at higher ages, and remarks that a person who has the potentiality of living to the age of 80 years has a force of life which is more or less independent of environment. Sir George Savage[49] often noted that of two aged members of the same family one was sober the other intemperate; and a good many centenarians have taken alcohol in quantities that would be too much for ordinary people.