[789] A possible survival of a world-serpent myth may be found in "Da Derga's Hostel" (_RC_ xxii. 54), where we hear of Leviathan that surrounds the globe and strikes with his tail to overwhelm the world. But this may be a reflection of Norse myths of the Midgard serpent, sometimes equated with Leviathan. CHAPTER XVI. SACRIFICE, PRAYER, AND DIVINATION. The Semites are often considered the worst offenders in the matter of human sacrifice, but in this, according to classical evidence, they were closely rivalled by the Celts of Gaul. They offered human victims on the principle of a life for a life, or to propitiate the gods, or in order to divine the future from the entrails of the victim. We shall examine the Celtic custom of human sacrifice from these points of view first. Cæsar says that those afflicted with disease or engaged in battle or danger offer human victims or vow to do so, because unless man's life be given for man's life, the divinity of the gods cannot be appeased.[790] The theory appears to have been that the gods sent disease or ills when they desired a human life, but that any life would do; hence one in danger might escape by offering another in his stead. In some cases the victims may have been offered to disease demons or diseases personified, such as Celtic imagination still believes in,[791] rather than to gods, or, again, they may have been offered to native gods of healing. Coming danger could also be averted on the same principle, and though the victims were usually slaves, in times of great peril wives and children were sacrificed.[792] After a defeat, which showed that the gods were still implacable, the wounded and feeble were slain, or a great leader would offer himself.[793] Or in such a case the Celts would turn their weapons against themselves, making of suicide a kind of sacrifice, hoping to bring victory to the survivors.[794] The idea of the victim being offered on the principle of a life for a life is illustrated by a custom at Marseilles in time of pestilence. One of the poorer classes offered himself to be kept at the public expense for some time. He was then led in procession, clad in sacred boughs, and solemnly cursed, and prayer was made that on him might fall the evils of the community. Then he was cast headlong down. Here the victim stood for the lives of the city and was a kind of scape-victim, like those at the Thargelia.[795]