In many simpler animals, almost all behavioral characteristics seemed to be instinctive. Tinbergen often spoke of sticklebacks as if they were machines that automatically reacted to stimuli. Using a modern figure of speech, ethologists believed that each species was "hardwired" to perform characteristic innate forms of behavior. Even in humans, which Tinbergen also studied, one could find some simple, instinctive behavioral characteristics. For example, breast-sucking seems to be an instinct that is automatically done by all newborn infants. Perhaps even complex forms of learned behavior had evolved from such instincts.