Originating in the work of Ernst Haeckel and Wilhelm Preyer, and advanced by a Prussian embryologist, Wilhelm Roux, the idea of struggle for existence between body parts helped to establish a framework, in which population cell dynamics rather than a predefined harmony guides adaptive changes in an organism. Intended to provide a causal-mechanical view of functional adjustments in body parts, this framework was also embraced later by early pioneers of immunology to address the question of vaccine effectiveness and pathogen resistance. (...) As an extension of these early efforts, Elie Metchnikoff established an evolutionary vision of immunity, development, pathology, and senescence, in which phagocyte-driven selection and struggle promote adaptive changes in an organism. Despite its promising start, the idea of somatic evolution lost its appeal at the turn of the twentieth century giving way to a vision, in which an organism operates as a genetically uniform, harmonious entity. (shrink)