"On Germinal Selection as a Source of Definite Variation" by August Weismann is a scientific publication written in the late 19th century. The work discusses the theory of germinal selection as a mechanism of evolution, aiming to clarify and expand upon the principles of natural selection originally proposed by Darwin and Wallace. Weismann seeks to argue that variation and adaptation arise not purely from random chance but through underlying processes connected to the germinal elements of organisms, which can direct the course of evolution. The opening of this work presents the author's preliminary arguments and the context for his theory. Weismann introduces the concept of germinal selection, which posits an internal mechanism that influences the direction of variation in response to external environmental pressures. He discusses the criticisms of conventional selection theories and emphasizes the importance of exploring deeper biological mechanisms, arguing that understanding the role of germs and determinants is essential for explaining how useful variations arise and how organisms adapt to their environments over generations. The preface sets the stage for a detailed analysis of heredity, variation, and selection, showcasing Weismann's ambition to enhance the discourse on evolutionary biology. (This is an automatically generated summary.)