This essay by the Spanish Nobel Laureate, Santiago Ram n y Cajal (1852-1934), is a tour-de-force vision on the present and future of humanity. It constitutes a preface to a book by the Cuban physician-sociologist, Enrique Florencio Lluria (1863-1925). In "Superorganic Evolution," the relation of sociology to the natural sciences is at the epicenter. Cajal's preface became popular with the anarchist and the socialist press and was reprinted many times,...