This book essentially restated the argument of Life and Habit, stating that human variation, or ‘progress’, is due to ‘strokes of cunning – to a sense of need, and to study of the past and present which have given shrewd people a key with which to unlock the chambers of the future’. Butler believed that heredity was a mode of memory, and instinct was inherited memory. A world shaped by natural selection appeared to Butler to be ‘a world of chance and blindness’, which he simply could not accept, and he seized on what he saw as the wilful aspect of the Lamarckian mechanism of evolution to refute the apparent arbitrariness of the Darwinian vision.