Our ape-men forefathers had no obvious natural weapons in the struggle for survival in the

open. They had neither the powerful teeth nor the strong claws of the big cats. They could not 【B1】______ with the bear, whose strength, speed and claws 【B2】______ an impressive 'smallfire' weaponry. They could not even defend themselves 【B3】______ running swiftly 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 【B4】______ to failure and extinction. But they were 【B5】______ with enormous concealed advantages of a kind not possessed by any of their competitors.

In the search 【B6】______ the pickings of the forest, the ape-men had 【B7】______ efficient stereoscopic vision and a sense of colour that the animals of the 'grasslands did not possess. The ability to see clearly at close range permitted the ape-men to study practical problems in a way that lay far 【B8】______ the reach of the original inhabitants of the grassland. Good long- distance sight was 【B9】______ another matter. Lack of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the higher the viewpoint, the greater the range of sight so 【B10】______ they had had to do was climb a tree. Out in the open, however, this simple solution was not 【B11】______ . Climbing a hill would have helped, but in many places the ground was flat. The ape-men 【B12】______ 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 considerable disadvantages. It was extremely unstable and it meant that the already slow ape-men became slower still.【B13】______ , they persevered and their bone structure gradually became【B14】______ to the new, unstable position that【B15】______them the name Homo erectus, upright man.

【B1】

A．match

B．compare

C．rival

D．equal