The rowers and pilots had little to do beyond keeping the raft (as we will call the combination of boats and platform just described) in the middle of the stream, where the current was strongest. There was a good deal of drift-wood in the river, but it was far less troublesome than if their course had been up the stream. Dr. Bronson explained to the youths that Madeira means "wood," and the Madeira River, into which the Beni flows, was so named by the Portuguese in consequence of the great number of floating trees that were met by the early explorers. The Beni contributes more than its share of this floating material, as the forests extend far along its banks, which are constantly crumbling away through the action of the current. In many places the Beni resembles the Missouri, and seems to be subject to the same forces of nature. [Illustration: SOUTH AMERICAN RIVER SCENE.] Forests and pampas, pampas and forests, succeeded each other as the raft followed the course of this affluent of the mighty Amazon. Parrots and toucans and other birds flew among the trees, monkeys stared in astonishment, jumped from limb to limb, swung by feet and tail, and kept up a continual chattering as the raft floated by their haunts. Frank made note of the difference between the South American monkey and his Asiatic brother; he had never seen the latter using his tail for anything but ornamental purposes, while with the South American monkey it gave the advantage of an extra hand or foot. [Illustration: SOUTH AMERICAN MONKEY WITH PREHENSILE TAIL.] "The Asiatic monkey's tail is not prehensile," said the Doctor, "and all monkeys of South America have not this advantage. In the words of a famous naturalist," he continued, "all monkeys with prehensile tails are American, but all American monkeys do not have prehensile tails. The Asiatic monkey does not seem to have heard of such a thing, though some of the varieties of monkey in the far East occasionally use the tail in a bungling sort of way. Professor Wallace lived four years in South America, and in that time he saw twenty-one species of monkey, seven with prehensile and fourteen with non-prehensile tails. All the American monkeys are climbers, and live in the trees, while such is not the case in the old world." While they were talking on the subject of monkeys a most unearthly yell was heard in the forest to the right of the raft. Both the boys turned in amazement to Manuel, and asked what it was. [Illustration: HOWLING MONKEY.] "It's a guariba," said Manuel, "as the natives call it." "And what is a guariba?" Fred inquired. "A guariba is a howling monkey," the guide answered, "and that is the noise he makes. You can hear him a long distance, and he howls night and day without seeming to get tired of the amusement."