Élie Metchnikoff, sub-director of the Pasteur Institute, opens this 1908 volume with a bold claim: biological science can address the three chief evils of disease, old age, and death. The editor, P. Chalmers Mitchell, frames the work as a continuation of Metchnikoff's earlier The Nature of Man, describing how the author identifies 'disharmonies'—persistent structures or habits from our evolutionary past that no longer serve us. Rather than turning to religion or philosophy, Metchnikoff places his hope in laboratory research, particularly on the microbial life of the human gut.

Disharmonies and the Optimistic Turn

Mitchell's introduction sets up a central tension: humanity has long viewed life as evil, with pessimism pervading philosophy, literature, and folklore. Metchnikoff, however, locates the source of this gloom not in fate but in our inherited constitution. He coins the term 'disharmony' to describe qualities that were once adaptive but have become harmful as environments changed. This framework allows him to reframe aging and death as biological problems rather than metaphysical ones. The excerpts do not reveal how Metchnikoff resolves each disharmony, but the editor makes clear that the author sees science—not anodynes—as the path forward. The optimism is conditional, a hope for future generations rather than a guarantee for the present.

The Bulgarian Bacillus and the Fight Against Putrefaction

A substantial portion of the excerpt details Metchnikoff's investigation of soured milks from various cultures: yahourth, leben, prostokwacha, kephir, and koumiss. He identifies the Bulgarian bacillus as the most effective lactic ferment, producing 25 grammes of lactic acid per litre of milk without forming alcohol or acetone. Importantly, he notes that this bacillus does not attack albuminoids or fats, making it ideal for 'arresting putrefactions and pernicious fermentations' in the digestive tract. Yet he also acknowledges practical problems: the bacillus can impart a 'disagreeable taste of tallow,' and commercial yahourth may contain harmful microbes like the red torula. His solution is a pure culture method, sometimes combining the Bulgarian bacillus with a paralactic bacillus for better flavour. The recommended daily dose is 300 to 500 cubic centimetres of soured skimmed milk.

Laboratory Precision and Cultural Knowledge

Metchnikoff's approach blends fieldwork with bench science. He obtains specimens from Geneva, studies them at the Pasteur Institute, and cites colleagues such as Massol, Grigoroff, Cohendy, and Michelson. The chemical analysis by Bertrand and Weisweiler quantifies the bacillus's output with precision. Yet the text also reveals a global perspective: Metchnikoff compares ferments from Bulgaria, Egypt, Russia, and Central Asia, treating each as a source of potential knowledge. The editor notes that this volume is part of a series of 'provocative and educational' essays, suggesting that Metchnikoff intended not only to report findings but to shift how readers think about aging. The excerpts do not reveal whether his clinical recommendations were widely adopted, but they show a scientist deeply engaged with both the microscopic and the cultural dimensions of his subject.

Readers should approach this text as a document of its time—a time when the idea of manipulating gut flora to extend life was novel and controversial. Metchnikoff's optimism is grounded in specific laboratory results, not vague hope. Pay attention to how he moves between evolutionary theory, chemical analysis, and practical dietetics. The editor's framing suggests that the book is as much a philosophical argument as a scientific treatise, one that asks what it means to live well in a body shaped by a distant past.