Darwin was, like Copernicus, a one-idea man. Each had his "nuclear inspiration" early in life, and spent the rest of his life working it out—the ratio of inspiration to perspiration being heavily in favor of the second. Both lacked the many-sidedness, that universality of interest and amazing multitude of achievement in unrelated fields of research which characterised Kepler, Newton, Descartes, Franklin, Faraday, Maxwell, and hundreds of lesser but equally versatile geniuses. It is perhaps no coincidence that both Darwin and Copernicus, after the decisive turning point when their course was set, led a life of duty, devotion to task, rigorous self-discipline, and spiritual desiccation. It looks as if the artesian wells of their inspiration had been replaced by a mechanical water supply kept under pressure by sheer power of will.