It is not an infrequent occurence for individuals among both the Yahgans and Onas to be subject to sudden outbursts of furor and violence. At such times the individual will generally dash from the wigwam and rush wildly away, and will continue running until nearly or completely exhausted. The one afflicted may dash madly through the woods or sometimes climb up dangerous cliffs. At such times, however, it is the custom of some of the men to follow closely behind to see that harm does not come through injury against trees, stumbling, or falling from the cliffs. However, at such times they rarely touch the afflicted one except to prevent harm, and finally will lead him back to the camp, when the attack is over or when he is exhausted. While the attack occurs both among men and women, it seems to be more prevalent among men. The individuals in whom these attacks predominate are men in the prime of life, ranging from 25 to 35 years of age. These people are polygamous and as it is the custom for the old men to marry young girls, thus leaving the old women to the younger men, which in many instances causes a scarcity of women, it leaves a somewhat undesirable condition. In many instances the character of the attack confines itself to the mad rushing away, as above described, at other times attempts to injure or kill others are made. For instance, a rancher of Tierra del Fuego, was in the company of some Onas when suddenly a hatchet whizzed by him, barely missing his head, and buried itself in a log of the Indian shelter. This was the result of an attack which seized upon one of the Onas who was afflicted thus from time to time. The actual outburst in this case was sudden, although it is difficult to tell how long it might have been coming on in the form of brooding, which seems to be a premonitory phase of this condition.