The object of this volume is to raise the question: if we accept the Theory of Evolution as true in science, how should it modify the thought and action of a man who wishes to do his best in this world? The question is necessary because we find that different and inconsistent conclusions on the point have been reached by men speaking in the name of science and speaking with authority. These differences are due not to anything in science, but to certain extra-scientific assumptions. To test the worth of such assumptions is the work of philosophy; and this volume is accordingly an essay in philosophy. Science is but organised common sense. Science and Religion both claim to deal with realities. The realism of common sense, therefore, the form of philosophy to which both seem to point, is that which is set forth here.