The study shows that in almost every animal, from the lowest to the highest, from frog to man, a somewhat sudden change of the usual environment deprives that animal of its activities and its functions. If the change is not too intense and prolonged, the animal merges into the hypnoidal state in which the lost functions are restored. During this hypnoidal state the functions are weakened, the animal may be regarded in a state of invalidism, its reactions being enfeebled, practically speaking, paretic. Perhaps it is advisable to approach the phenomena from their more striking aspect. In seizing a triton, salamander, or frog, and stretching it on the table, one will observe with surprise that the animal remains in the same position given it. The most uncomfortable and bizarre position may be given to the limbs, and still the animal will not move. Testing the extremities one finds them rigid and resisting. Something similar we find in the hypnotic condition, when under the suggestion that the extremities are rigid, and they cannot be moved by the subject. The same can be done with a lobster and other animals of the same type. Everyone has heard of the _experimentum mirabile_ made by Kirchner in the seventeenth century. A rooster or hen is seized, the legs are tied with a string and the bird is put on the ground. A piece of chalk is passed over the beak,--the chalk tracing a line from beak to some distant point on the ground. When the bird is released, it remains in the same position. Some explain that the animal is kept prisoner, because it “imagines” that it is bound by the line of chalk. The chalk, however, is unnecessary. The animal may be seized, shouted in its ear, or kept down forcibly, and the same result will happen. This state has been termed by Preyer “cataplexy.” The phenomenon can also be produced in insects, in mollusca. Many investigators have been interested in this phenomenon. I have devoted a good deal of work to this condition which is also found in mammals in which it is induced by the fear instinct. In mammals, however, the state of cataplexy is not necessarily accompanied by rigidity, although it may be present, but there is a complete loss of voluntary activity. Horses tremble violently and become paralyzed at the sight of a beast of prey, such as a tiger or a lion. At the sight of a serpent, monkeys are known to be in such an intense state of fear that they are unable to move, and thus fall easy victims to the reptile.