The theory of the construction of Tables of Mortality offers no difficulty
when applied either to the members of a Society, or to a collection of
individuals, who can be traced so that all the necessary particulars as to
them are known. But in the population of a country or a town, several
incidental causes, such as epidemics and migrations, exercise an influence;
some of them on the total numbers of the population, and others on the
members living at certain ages. The mobility of a population or the constant
change taking place, from one residence to another, or from one country to
another, prevents us from following each individual or each group of
individuals from the moment of birth to that of death.