Small birds are remarkably good at building nests with such a clumsy organ as the beak, but we could not expect them to develop the manual skill and insight of an animal like the chimpanzee with a pair of hands. When biologists accepted Darwin's theory that men were descended from animals, they naturally tried to find some way of estimating intelligence in animals. In particular Kohler found that some chimpanzees showed a good deal of intelligence in the use of solid objects as tools. One of Yerkes's methods was to give the animals a choice of a number of boxes, say nine in a row, of which some were open and some shut. Coburn and Yerkes managed to teach two crows to choose the farthest open door on the right, but it was a slow business, and they never mastered the choice of the first from the left completely.