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 head-hunters 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.

“Miranha or Amajuaca cannibals,” said Challenger, jerking his thumb towards the reverberating wood.

“No doubt, sir,” Summerlee answered.  “Like all such tribes, I shall expect to find them of poly-synthetic speech and of Mongolian type.”

“Polysynthetic certainly,” said Challenger, indulgently.  “I am not aware that any other type of language exists in this continent, and I have notes of more than a hundred.  The Mongolian theory I regard with deep suspicion.”

“I should have thought that even a limited knowledge of comparative anatomy would have helped to verify it,” said Summerlee, bitterly.

Challenger thrust out his aggressive chin until he was all beard and hat-rim.  “No doubt, sir, a limited knowledge would have that effect.  When one’s knowledge is exhaustive, one comes to other conclusions.”  They glared at each other in mutual defiance, while all round rose the distant whisper, “We will kill you—­we will kill you if we can.”

That night we moored our canoes with heavy stones for anchors in the center of the stream, and made every preparation for a possible attack.  Nothing came, however, and with the dawn we pushed upon our way, the drum-beating dying out behind us.  About three o’clock in the afternoon we came to a very steep rapid, more than a mile long—­the very one in which Professor Challenger had suffered disaster upon his first journey.  I confess that the sight of it consoled me, for it was really the first direct corroboration, slight as it was, of the truth of his story.  The Indians carried first our canoes and then our stores through the brushwood, which is very thick at this point, while we four whites, our rifles on our shoulders, walked between them and any danger coming from the woods.  Before evening we had successfully passed the rapids, and made our way some ten miles above them, where we anchored for the night.  At this point I reckoned that we had come not less than a hundred miles up the tributary from the main stream.