The function of fear is quite clear. Fear is the guardian instinct of life. The intensity of the struggle for existence and the preservation of life of the animal are expressed in the instinct of fear. The fear instinct in its mild form, when connected with what is strange and unfamiliar, or with what is really dangerous to the animal, is of the utmost consequence to the life existence of the animal. What is strange and unfamiliar may be a menace to life, and it is a protection, if under such conditions the fear instinct is aroused. Again, it is of the utmost importance in weak animals, such as hares or rabbits, to have the fear instinct easily aroused by the slightest, strange stimulus: the animal is defenseless, and its refuge, its safety, is in running. The unfamiliar stimulus may be a signal of danger, and it is safer to get away from it; the animal cannot take chances. On the other hand, animals that are too timid, so that even the familiar becomes too suspicious, cannot get their food, and cannot leave a progeny,--they become eliminated by the process of natural selection. There is a certain amount of trust that nature demands even of its most defenseless and timid children. Animals in whom the fear instinct can be aroused to a high degree become paralyzed and perish. Under such conditions the fear instinct not only ceases to be of protective value, but is the very one that brings about the destruction of the animal possessed by it. Intense fear paralyzes the animal. “One of the most terrible effects of fear,” says Mosso, “is the paralysis which allows neither of escape nor defense. Not all the phenomena of fear can be explained on the theory of natural selection. In their extreme degree they are morbid phenomena, indicating imperfection of the organism. One might almost say that nature had not been able to find a substance for brain and spinal cord which should be extremely sensitive, and yet should never, under the influence of exceptional or unusual stimuli, exceed in its reactions those physiological limits which are best adapted to the preservation of the animal.” Mosso quotes Haller to the effect that “phenomena of fear common to animals are not aimed at the preservation of the timid, but at their destruction.” The fear instinct is no doubt one of the most fundamental and one of the most vital of animal instincts, but when it rises to an extreme degree, or when associated with familiar instead of strange and unfamiliar objects, then we may agree with Haller that the phenomena are not aimed at the preservation of the animal, but at its destruction; or, as Darwin puts it, are of “disservice to the animal.” This is just what is found in the case of psychopathic or neurotic affections. The fear instinct, when aroused and cultivated in early childhood, becomes associated in later life with particular events, objects, and special states.