Animal Hypnosis.—Many animals, in response to particular stimuli, fall into a remarkable state which has frequently been compared to the physiological phenomenon of human hypnosis.
A so-called "fear-rigor" is known in the brittle-stars. In higher Crustacea a "cramp-reflex" can easily be called forth. As early as the middle of the last century the so-called "magnetism" of river crayfishes was known. By stroking the animals in a direction from head to tail they can be rendered motionless. But holding them on the back will suffice, or making them stand on their head with the claws bent outwards. The "feigning death" of many insects, particularly beetles, comes into the same category. The "cataleptic rigor" of the stick-insect, Dixippus morosus, is par ticularly remarkable. Vertebrates, too, can be put into a state of rigor. It usually suffices to place the animal on its back and to hold it there for a while, at the same time preventing all move ment. Not only fishes, frogs, snakes, crocodiles, but also all sorts of birds, rodents, goats, pigs and dogs can be rendered temporarily motionless.
As to the meaning of these phenomena, in the invertebrates inborn, mostly useful, reflexes are involved, by which the animals are able on occasion to escape from their enemies. In the verte brates the state of rigor is probably the result of peculiar inhibi tions in the central nervous system which are brought into action by the cessation of all movement. Whereas the duration of the state of rigor may in the lower animals be considerable, in vertebrates it is the shorter as the brain of the particular animal is more highly developed. The lengths of time during which certain animals remain in a state of rigor are as follows : a stick insect, up to 41 hours; a crayfish, to 4 minutes; a frog, several hours; a goose or a hen, about 2 hour; a rabbit up to 6 minutes; a dog, usually only i minute. At the end of this interval, or of a shorter one, the animal wakes up spontaneously. But this can be brought about at an earlier time by weak external stimuli. From the physiological phenomena occurring during rigor, which have been noted in numerous investigations, it is certain that this state is something quite different from human hypnosis. It is only lately that hens and dogs have been put into a condition of true hypno sis, i.e., into a state of rigor exactly corresponding to that assumed in human hypnosis. The means employed to induce this state were the same as those customary for human beings. A shining object is held before a hen, or when sitting on the experimenter's hand the bird is moved slowly up and down. Forthwith the animal sinks into sleep. The state of hypnosis is so profound that the bird can be lifted up by one leg and set down again, without being wakened up.
There exist analogies in animals, too, for the mass-psychosis observable in human beings under certain circumstances. Usually it is an aimless wild flight of a group of animals without any reasonable cause. Travellers describe such stampedes of im mense troops of ungulates such as antelopes, zebras and so forth, in the tropical steppe country. Stampedes of the half-wild horses which live in huge herds on the South American pampas are highly undesirable happenings for the owners of the animals. The cause of these occurrences is the action of a purposeful instinct by which a warning signal from a leader or other member of the company causes all the animals to take to flight. As soon, however, as the resulting general excitement and flight movement increases out of all proportion to the cause, we have a definite case of mass-psychosis.