Concepts on the Causal Genesis of Human Intestinal Tumors The action of chemical substances has a significant role in the genesis of human tumors. It is assumed that most human tumors are induced by exogenous chemical noxae (Schmahl, 1970;Heidelberger, 1975; and others). In the course of intensive efforts to discover the principles of tumor etiology, important insights have resulted in recent years. These are that chemical carcinogens do not only arise as products of our tech nologic...