As the reign of behaviorism continued, the limits of the paradigm became more evident. Animals kept behaving in ways that could not be explained by behaviorist theory alone. For example, Skinner had thought that any animal could be taught any behavior with the appropriate reinforcement schedule. But this did not turn out to be the case. The same behavior was learned easily by some animals, with difficulty by others, and not at all by still others. Rats could easily learn to press a bar for food, while cats would do so only with difficulty. These findings suggest that the genetics of each animal species set the parameters of what could and could not be learned. There were limits to what could be taught.