Mental disease in captive animals offers a fascinating field for study— now chiefly speculative. Many conditions are present to produce it—all the conditions that cause prison psychoses in man. And many if not all the insane and perverted acts of animals have their counterpart in the human. Masturbation is very common in mammals. Eating of their own feces, coprophagy, is not infrequent, and is especially notable in one of the highest types—the Chimpanzee. Perhaps occasionally coprophagy may be due to some defect in diet. There is no instinctive disgust at excrement in the lower animals any more than there is in the uneducated child. Nevertheless, eating feces cannot be healthful, and probably does not occur in nature; and occurs only in the human with mental disease. Some of the insane acts of animals if prevalent in the wild would probably cause the extinction of the race. Such are killing of the young by the father and by the mother; killing of the female by the male, usually during rutting time, in some instances reminding one of Sadism in man. This kind of sexual killing does not often occur in the wild. The female has a better chance to escape, and the male probably does not feel so inclined to damage her when he chooses her himself as when she is chosen for him by his gaoler. When the mother devours her young it is usually shortly after birth. I have, however, the record of a Jungle cat (_Felis chaus_) who ate two of her kittens when they were seven weeks old. Some of the domestic animals devour their young; the sow often does it, and occasionally the bitch. Sucking, gnawing and eating parts of itself or of a cage-mate is not uncommon in a variety of animals. Bears lick their paws until they are sore; a monkey may gradually gnaw away its tail from the tip to the body; an ocelot (_Felis chibigonazon_) bit open his scrotum and devoured his testicle; a Tasmanian devil (_Sarcophilus ursinus_) bit off one of his front feet at the wrist; a monkey may gradually gnaw away its fingers; and numerous other self-inflicted mutilations occur. Often there is a local irritating cause, as skin disease, lice, or freezing. But in many cases no local cause can be found, as no local cause can be found for thumb-sucking or nail-biting in the human. The surgery done by monkeys on their frozen fingers and toes is interesting. After the flesh has sloughed the monkey bites off the protruding phalanges, apparently without pain, so that satisfactory well-covered stumps are made. Animals often mutilate their cage-mates in an amicable way as distinguished from fighting. A bear may lick its mate’s ears until the hair and skin are gone. A monkey may eat its mate’s tail or patches of its skin, the victim lying placidly while the process goes on. Many animals are addicted to perverted acts on their own or their mate’s sexual organs.