The Prolongation of Life, the main thesis is carried further, and a number of criticisms and objections are met. The latter, so far as they relate to technical details, I need say nothing of here, as Metchnikoff and his staff at the Pasteur Institute are the most skilled existing technical experts on these matters, but I cannot refrain from a word of comment on the brilliant treatment of the objection to the suggested amelioration of human life that it considered only the individual and neglected the just subordination of the individual to society. In the sixth Part of this volume, Metchnikoff discusses the relation of the individual to the species, society or colony, from the general point of view of comparative biology, and shows that as organisation progresses, the integrity of the individual becomes increasingly important. Were orthobiosis, the normal cycle of life, attained by human beings, there still would be room for specialisation of individuals and for differentiation of the functions of individuals in society, but instead of the specialisation and differentiation making individuals incomplete throughout their whole lives, they would be distributed over the different periods of the life of each individual. As these lines are intended to be an introduction, not a commentary, I will now leave the reader to follow the argument in the book itself.

Ilya Ilyich Mechnikov (Russian: , also written as Élie Metchnikoff; 15 May [O.S. 3 May] 1845 15 July 1916) was a Ukrainian zoologist best known for his pioneering research in immunology. In particular, he is credited with the discovery of phagocytes (macrophages) in 1882. This discovery turned out to be the major defence mechanism in innate immunity. He and Paul Ehrlich were jointly awarded the 1908 Nobel Prize in Physiology or Medicine "in recognition of their work on immunity". He is also credited by some sources with coining the term gerontology in 1903, for the emerging study of aging and longevity. He established the concept of cell-mediated immunity, while Ehrlich established the concept of humoral immunity. Their works are regarded as the foundation of the science of immunology. In immunology, he is given an epithet the "father of natural immunity".