We may call this possibility of forming habits _plasticity_. Inanimate objects such as iron, rocks, sulphur, oxygen, etc., have no plasticity. Plants have very little possibility of forming habits. Lower animals have somewhat more, and higher animals still more, while man has the greatest possibility of forming habits. This great possibility of forming habits is one of the main characteristics of man. Let us illustrate the contrast between man and inanimate objects by an example. If sulphur is put into a test tube and heated, it at first melts and becomes quite thin like water. If it is heated still more, it becomes thick and will not run out of the tube. It also becomes dark. Sulphur _always_ does this when so treated. It cannot be taught to act differently. Now the action of sulphur when heated is like the action of a man when he turns to the right upon meeting a person in the street. But the man has to acquire this habit, while the sulphur does not have to learn its way of acting. Sulphur always acted in this way, while man did not perform his act at first, but had to learn it by slow repetition. Everything in the world has its own peculiar nature, but man is unique in that his nature can be very much changed. To a large extent, a man is _made_, his nature is _acquired_. After we become men and women, we have hundreds and thousands of tendencies to action, definite forms of action, that we did not have when young. Man's nature might be said to consist in his tendencies to action. Some of these tendencies he inherits; these are his instincts. Some of these he acquires; these are his habits. =What Habits Do for Us.= We have found out what habits are like; let us now see what they do for us. What good do they accomplish for us? How are we different after forming a habit from what we were before? We can best answer these questions by a consideration of concrete cases. Typewriting will serve very well the purpose of illustration. We shall give the result of an actual experiment in which ten university students took part. During their first half hour of practice, they wrote an average of 120 words. At the end of forty-five hours of practice, they were writing an average of 680 words in a half hour. This was an increase of speed of 560 per cent. An expert typist can write about 3000 words in a half hour. Such a speed requires much more than forty-five hours practice, and is attained by the best operators only. [Illustration: FIGURE III.--LEARNING CURVES The upper graph shows the improvement in speed of a group of students working two half hours a day. The lower curve shows the improvement of a group working ten half-hours a day.]