JACK LONDON lived at a time when a dramatically new set of ideas, growing out of the theory of evolution, was changing the course of men’s thinking. These ideas stimulated, frustrated, and tantalized London all his adult years. Charles Darwin and Herbert Spencer, messiahs of the new creed, became his intellectual mentors, along with Friedrich Nietzsche and Karl Marx. It might be said that London’s very real private struggle with life—which he dramatized in stories so arresting and exciting that they are still read over the world—became for him an epitome of the Darwinian Struggle for Existence, his success an example of the Spencerian Survival of the Fittest.

It is not easy for people today, who have lived with accelerating change through two-thirds of the twentieth century, to grasp how revolutionary and shattering were Darwin’s and Spencer’s ideas. Before they burst upon the intellectual horizon it was, for example, generally held that the world had been created in precisely the year 4004 B.C. and that the various species of flora and fauna were immutable: a rose, or a horse, or a man had always been the same creature, although variations or developments within each kind were possible. Then came Darwin in 1859 to propose that the earth was many millions of years old and that all extant species had evolved from a common beginning in the sea at some remote moment of time. Design and order had not presided over this evolution, either; it took place through the accumulation of infinitesimal and accidental variations. Millions upon millions of individuals (in whatever species) were wasted in the struggle for existence in which the slightly superior variation managed to survive and reproduce itself. The first reaction to this great theory was outrage. Darwin was denounced from countless Christian pulpits by ministers who accused him of maintaining that man descended from monkeys—although this was one point that he did not urge in his Origin of Species. Even so, the implication was unmistakable, and the very foundations of religion seemed to be threatened.

If Darwin was the scientist of evolution, Herbert Spencer was its philosopher. He worked from 1860 to 1903 on the many volumes of his great Synthetic Philosophy, a work that undertook a new synthesis of knowledge based on a new guiding idea, which was, of course, evolution. Spencer asserted that evolution is the fundamental law of social as well as physical process: from simple and relatively uniform materials evolve increasingly complex and specialized structures. He contended that the more complex forms, whether individual creatures or social organizations, are the more stable— and thus he saw the social struggle for existence as leading up to the ultimately perfect and stable society. The evils of child labor, poverty, unemployment, and industrial warfare which were rampant in Western Europe and America were justified because they were the means to that perfect society. The fit would survive. Every social and industrial violence, every outrage caused by competition, was beatified with...