The Lost World by Arthur Conan Doyle. Chapter 8 "The Outlying Pickets Of The New World". Originally published as a serialized story in The Strand magazine from April to October 1912 vol. 43 and 44.

  For two days we made our way up a good-sized river some hundreds of yards broad, and dark in color, but transparent, so that one could usually see the bottom. The affluents of the Amazon are, half of them, of this nature, while the other half are whitish and opaque, the difference depending upon the class of country through which they have flowed. The dark indicate vegetable decay, while the others point to clayey soil. Twice we came across rapids, and in each case made a portage of half a mile or so to avoid them. The woods on either side were primeval, which are more easily penetrated than woods of the second growth, and we had no great difficulty in carrying our canoes through them. How shall I ever forget the solemn mystery of it? The height of the trees and the thickness of the boles exceeded anything which I in my town-bred life could have imagined, shooting upwards in magnificent columns until, at an enormous distance above our heads, we could dimly discern the spot where they threw out their side-branches into Gothic upward curves which coalesced to form one great matted roof of verdure, through which only an occasional golden ray of sunshine shot downwards to trace a thin dazzling line of light amidst the majestic obscurity. As we walked noiselessly amid the thick, soft carpet of decaying vegetation the hush fell upon our souls which comes upon us in the twilight of the Abbey, and even Professor Challenger's full-chested notes sank into a whisper. Alone, I should have been ignorant of the names of these giant growths, but our men of science pointed out the cedars, the great silk cotton trees, and the redwood trees, with all that profusion of various plants which has made this continent the chief supplier to the human race of those gifts of Nature which depend upon the vegetable world, while it is the most backward in those products which come from animal life. Vivid orchids and wonderful colored lichens smoldered upon the swarthy tree-trunks and where a wandering shaft of light fell full upon the golden allamanda, the scarlet star-clusters of the tacsonia, or the rich deep blue of ipomaea, the effect was as a dream of fairyland. In these great wastes of forest, life, which abhors darkness, struggles ever upwards to the light. Every plant, even the smaller ones, curls and writhes to the green surface, twining itself round its stronger and taller brethren in the effort. Climbing plants are monstrous and luxuriant, but others which have never been known to climb elsewhere learn the art as an escape from that somber shadow, so that the common nettle, the jasmine, and even the jacitara palm tree can be seen circling the stems of the cedars and striving to reach their crowns. Of animal life there was no movement amid the majestic vaulted aisles which stretched from us as we walked, but a constant movement far above our heads told of that multitudinous world of snake and monkey, bird and sloth, which lived in the sunshine, and looked down in wonder at our tiny, dark, stumbling figures in the obscure depths immeasurably below them. At dawn and at sunset the howler monkeys screamed together and the parrakeets broke into shrill chatter, but during the hot hours of the day only the full drone of insects, like the beat of a distant surf, filled the ear, while nothing moved amid the solemn vistas of stupendous trunks, fading away into the darkness which held us in. Once some bandy-legged, lurching creature, an ant-eater or a bear, scuttled clumsily amid the shadows. It was the only sign of earth life which I saw in this great Amazonian forest.

And yet there were indications that even human life itself was not far from us in those mysterious recesses. On the third day out we were aware of a singular deep throbbing in the air, rhythmic and solemn, coming and going fitfully throughout the morning. The two boats were paddling within a few yards of each other when first we heard it, and our Indians remained motionless, as if they had been turned to bronze, listening intently with expressions of terror upon their faces.

"What is it, then?" I asked.

"Drums," said Lord John, carelessly; "war drums. I have heard them before."

"Yes, sir, war drums," said Gomez, the half-breed. "Wild Indians, bravos, not mansos; they watch us every mile of the way; kill us if they can."

"How can they watch us?" I asked, gazing into the dark, motionless void.

The half-breed shrugged his broad shoulders.

"The Indians know. They have their own way. They watch us. They talk the drum talk to each other. Kill us if they can."

By the afternoon of that day--my pocket diary shows me that it was Tuesday, August 18th--at least six or seven drums were throbbing from various points. Sometimes they beat quickly, sometimes slowly, sometimes in obvious question and answer, one far to the east breaking out in a high staccato rattle, and being followed after a pause by a deep roll from the north. There was something indescribably nerve-shaking and menacing in that constant mutter, which seemed to shape itself into the very syllables of the half-breed, endlessly repeated, "We will kill you if we can. We will kill you if we can." No one ever moved in the silent woods. All the peace and soothing of quiet Nature lay in that dark curtain of vegetation, but away from behind there came ever the one message from our fellow-man. "We will kill you if we can," said the men in the east. "We will kill you if we can," said the men in the north.