The author compiles and analyzes August Weismann's evolving heredity theories, first summarizing earlier positions, then tracing later modifications such as amphimixis and the germ-plasm concept, and setting out Weismann's 1891 theory of heredity and its evolutionary implications. He evaluates logical coherence, contrasts Weismann's ideas with alternative views like Darwin's pangenesis and Galton's proposals, and discusses methodological limits by deferring the empirical question of whether acquired traits are inherited to another volume. The volume closes with an up-to-date critique and appendices addressing technical issues of germ-plasm and telegony.