THE ACQUISITION OF NEW MODES OF RESPONSE. Expressed in less technical language this means simply that human beings can learn by experience, and that they tend to repeat actions they have once learned. Where an animal is perfectly adjusted to its environment, all stimuli issue in immediate and nicely adjusted responses. This happens only where the environment is very simple and stable, and where in consequence no complexity of structure or action is necessary. In the clam and the oyster, and in some of the lower vertebrates, perhaps, instinctive activity is almost exclusively present. But in the case of man, so complicated are the situations to which he is exposed that random instinctive responses will not solve his problems. He must, as with his highly modifiable nervous system he can, acquire new modes of response which will, in the complexity of new situations serve as effectively as his original tendencies to act would serve him in a simpler and stabler environment. A human being in a modern city cannot live by instinct alone; he must acquire an enormous number of habits to meet the variety of complex situations he meets in daily life. A monkey exists with fairly fixed native tendencies to act. But civilization could never have developed if in man new ways could not be acquired to meet new situations, and if these new ways could not be retained and made habitual in the individual and the race. TRIAL AND ERROR AND DELIBERATE LEARNING. Whenever, as happens a large number of times daily in the life of the average man, old ways of response, inborn or formerly acquired, are inadequate to meet a new situation, there are two methods of acquiring a new and more adequate response. One is the method of trial and error, already discussed, whereby animals and humans try every possible instinctive response to a situation until one brings satisfaction and is retained as a habitual reaction when that situation recurs. The other is a delay in response, during which delay reflection, a consideration of possible alternatives, and a conscious decision, take place. The technique of this latter process will be discussed more specifically in the next chapter. Whether acquired by trial and error, or through reflection, learned acts are, the first time they are performed, frequently imperfect, only partly effective, and performed with some difficulty. With successive repetitions their performance becomes more rapid, more immediate, and more adjusted to the specific situation to be met. And as they become more familiar responses to familiar stimuli they cease to be conscious at all. They are performed with almost as little difficulty or attention as normal breathing.