Our ape-men forefathers had no obvious natural weapons in the struggle for survival in the open. They had neither the (1) teeth nor the strong claws of the big cats. They could not (2) with the bear, whose strength, speed and claws (3) an impressive "small fire" weaponry(武器). They could not even defend themselves by running (4) like the horses, zebras or small animals. If the ape-men had attempted to compete on those terms in the open, they would have been (5) to failure and extinction. But they were (6) with enormous concealed advantages of a kind not possessed by any of their competitors. In the search (7) the pickings of the forest, the ape-men had (8) efficient stereoscopic(有立体感的) vision and a sense of colour that the animals of the grasslands did not possess. The ability to see clearly at close (9) permitted the ape-men to study practical problems in a way that lay far (10) the reach of the original inhabitants of the grassland. Good long-distance sight was (11) another matter. Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the higher the (12) , the greater the range of sight—so all they had had to do was climb a tree. Out in the open, however, this solution was not (13) . Climbing a hill would have helped, but in many (14) the ground was flat. The ape-men (15) the only possible solution. They reared up as high as possible on their hind limbs and began to walk upright. This vital change of physical position brought about (16) disadvantages. It was extremely (17) and it meant that the already slow ape-men became slower still. (18) , they persevered and their bone structure gradually became (19) to the new, unstable position that (20) them the name Homo erectus, upright man.