While the gradual decay of the bodily forces and the senile degeneration of the organs must necessarily cause the death of the soundest organism in the end, the great majority of men pass away through illness long before this normal term of life is reached. The external causes of this are the attacks of enemies and parasites, accidents, and unfavorable conditions of life. These cause changes in the tissues and their component cells, which first occasion the partial death of particular sections, and then the total death of the whole individual. The modifications of the living matter which produce disease and premature death are called _necrobioses_. They consist partly of _histolyses_--that is to say, degeneration of the cells by atrophy, dissolution, withering (mortification), or colliquation; and partly of _metaplasmosisms_, or metamorphoses of the plasm--fatty, mucous, chalky, or amyloid metamorphoses of the cells. It was the great merit of Rudolph Virchow that he proved, in his epoch-making _Cellular Pathology_ (1858), that all diseases in man and other organisms may be reduced to such modifications of the cells which make up the tissues. Hence disease, with its pain, is a physiological process, a life under injurious and dangerous conditions. As in all normal vital phenomena, so in abnormal or pathological, the ultimate ground must be sought in the physical and chemical processes in the plasm. Pathology is a part of physiology. This discovery has cut the ground from under the older notion of disease as a special entity, a devil, or a divine punishment.