All day the drums rumbled and whispered, while their menace reflected itself in the faces of our colored companions. Even the hardy, swaggering half-breed seemed cowed. I learned, however, that day once for all that both Summerlee and Challenger possessed that highest type of bravery, the bravery of the scientific mind. Theirs was the spirit which upheld Darwin among the gauchos of the Argentine or Wallace among the headhunters of Malaya. It is decreed by a merciful Nature that the human brain cannot think of two things simultaneously, so that if it be steeped in curiosity as to science it has no room for merely personal considerations. All day amid that incessant and mysterious menace our two Professors watched every bird upon the wing, and every shrub upon the bank, with many a sharp wordy contention, when the snarl of Summerlee came quick upon the deep growl of Challenger, but with no more sense of danger and no more reference to drum-beating Indians than if they were seated together in the smoking room of the Royal Society’s Club in St. James’s Street. Once only did they condescend to discuss them.