填空题

Our ape-men forefathers had no obvious natural weapons in the
struggle for survival in the strong claws of the big cats. They had neither the
powerful teeth nor the strong claws, and could not rival with the bear,
whose strength and speed provided an impressively "small-fire" weaponry. 67. ______.
They could not even defend themselves by running swiftly like horses. If the
ape-men attempted to compete on those terms in the open, they would have 68. ______.
been doomed to failure and extinction. But they were born with enormous concealing 69. ______.
advantages of a kind not possessed by any of its competitors. 70. ______.
In the search for the pickings of the forest, the ape-men were developed 71. ______.
efficient short-distance vision and a sense of color that the animals of the
grasslands did not possess. The ability to see dearly at dose range permitted
them to study practical problems in a way that lay far from the reach of 72. ______.
original inhabitants of the grassland. Good long-distance sight was another
matter. Lack of short-distance vision had not been a problem for forest- 73. ______.
dwelling apes and monkeys because of the higher the viewpoint, the greater 74. ______.
the range of sight. So everything they had to do was climb a tree. But out 75. ______.
in the open this simple solution was available. Climbing a hill would have 76. ______.
helped, but in many places the ground was flat. The ape-men had only 77. ______.
possible solution. They stood up on their hind limbs (后肢) and began to
walk upright.