I cannot better introduce the subject of this paper than by a quotation from Mr. Morgan’s Preface to his Tables of the Experience of the Equitable Society. He writes as follows :—

“In a body of lives of the same age, all selected as healthy from the general mass of mankind, it is obvious that the rate of mortality must be considerably less for the first ten or twenty years after selection, than amongst those from whom they are thus chosen. As, however, these selected lives advance in age, their general health and the rate of mortality amongst them will naturally approximate to the common standard. This approximation cannot be accurately estimated, if the observations be taken from a blended mass of lives, selected at the age of twenty, for instance, which have attained the age of fifty, and of lives selected at the age of fifty. The rate of mortality will for the former class be less, for the latter greater, than that which occurs in either class separately.