Both birds and mammals often kill their mates when the mate is sick, or “down” from injury or disease. All animals hate sickness and death, and show their dislike by attacking or shunning it. Birds may get on happily together for months until one becomes sick, and as he crouches in a corner with ruffled feathers the others pick on him and finish him. The same is true of mammals, the sick one being horned or tramped to death by the mate with whom he had formerly been on most friendly terms. The keeper often reports an animal “killed by its mate,” whereas the mate has only given the _coup de grace_. This brutality is not universal. Rarely a parrakeet will stand guard over his sick and dying mate; and we have seen a ratel—of a ferocious family—stand guard over and resist the removal of his sick companion. The diagnosis of disease in wild animals is unsatisfactory; usually impossible; clinical study as we know it in the human is impossible. We know that the animal is sick, but not why. A certain group of symptoms accompanies all diseases—dull, rough coat or feathers, refusal to eat, weakness in the hind quarters, and finally getting down. They rarely show symptoms of pain—or at least we cannot read the symptoms. The pain of acute pancreatitis in man is violent, yet many animals die with it and we cannot tell that they suffer. Animals do not suffer as much as the human, and they stand the ravages of disease better than the human. At autopsy we often wonder how the animal could have lived with the conditions that are found. A monkey may be apparently well until a few days before his death, though his lungs and abdominal organs may be a mass of tubercle. A small red howler monkey (_Alonata seniculus_) was in apparent good health, playful and lively until twenty-four hours before his death from acute pancreatitis, though his stomach and intestines contained fifty-one nematode worms, some of which were eight inches in length.