.  Monkeys and chimpanzees, although they are weaker and less fierce than many other animals, possess brains which are far along the evolutionary road as any creature other than man. Birds can perform marvels of aerobatics, they can catch insects on the wing with unparalleled skill, navigate in a remarkable manner half round the world and back—but they can cannot think and reason. In technical terms it can be said that they are lacking in insight. The abilities which they do possess are built-in instincts derived from their genetic inheritance. Monkeys, on the other hand, can reason. They can easily remember a lighted door indicating the presence of food. They can remember what kind of food they are looking for. A monkey set the problem of reaching a banana, say, hung high up in its cage, can work out a system for getting it even if it involves piling up boxes to stand on and then knocking down the banana with a stick. A charming story is told about the psychologist Wolfgang Kobler, who had provided various boxes and other apparatuses by which he proposed to test a chimpanzee's ability to think out a method of reaching a fruit hung nine feet in the air. The animal looked about it and sized up the problem. Then it took Kobler by the hand, led him to a position immediately under the banana, jumped upon to his shoulder and reached it down from there.
    But evolution, although it has brought monkeys to a remarkable degree of cleverness, has stopped short at a crucial ability, the possession of which places man at a clearly superior level. Their minds cannot cope with abstract ideas. For example, an ape can be taught to fill a can with water from a barrel and take the can of water to put out a fire so that it can reach into a box and get food. But if the whole set-up is arranged on a raft the animal will continue to draw its water only from the barrel. It cannot grasp that any water, taken more, conveniently, say, from the pond on which the raft is floating, will putout the fire just as well. The abstract idea that water puts out fire is beyond it. The abstract idea that water puts out fire is beyond it.1.  Birds owe their remarkable abilities to ______.