From the point of view of physiology, the process of habit-formation consists in securing a particular nerve coupling, establishing a particular nerve path, so that a definite form of stimulation will bring about a definite form of response. A nerve tendency is simply the likelihood that a stimulus will take a certain course rather than any other. This likelihood is brought about by getting the stimulus to take the desired route through the nervous system to a group of muscles and to continue following this route. The more times it passes the same way, the greater is the probability that at any given time the stimulus will take the accustomed route and bring about the usual response. At first any sort of action is possible. A nerve stimulus can take any one of the many routes to the different muscles. By chance or by conscious direction, the stimulus takes a certain path, and by repetition we fix and make permanent this particular route. This constitutes a nerve tendency or habit. =Plasticity.= Our discussion should have made it clear that habit is acquired nature, while instinct is inherited nature. Habit is acquired tendency while instinct is inherited tendency. The possibility of acquiring habits is peculiarly a human characteristic. While inanimate things have a definite nature, a definite way of reacting to forces which act upon them, they have little, if any, possibility of varying their way of acting. Water might be said to have habits. If one cools water, it turns to ice. If we heat it, it turns to steam. But it _invariably_ does this. We cannot teach it any different way of acting. Under the same conditions it always does the same thing. Plants are very much like inanimate things. Plants have definite ways of acting. A vine turns around a support. A leaf turns its upper surface to the light. But one cannot teach plants different ways of acting. The lower forms of animals are somewhat like plants and inanimate objects. But to a very slight extent they are variable and can form habits. Among the higher animals, such as dogs and other domestic animals, there is a greater possibility of forming habits. In man there are the greatest possibilities of habit-formation. In man the learned acts or habits are many as compared to the unlearned acts or instincts; while among the lower animals the opposite is the case--their instincts are many as compared to their habits.