_Progressive Character of Death. Accidental Death._—We have seen that in general the disappearance of the characteristics of vitality is not instantaneous, at least in the natural course of things, in complex organisms. It is the end of a more or less rapid process. But death is not instantaneous in the isolated anatomical element any more than it is in the protozoan or protophyte. We must have recourse to very violent devices of destruction to kill the cell at a blow, to leave absolutely nothing of its organization existing. The protoplasm of yeast when violently crushed by Büchner still possessed the power of secreting soluble ferments. A powerful action, a very high temperature, is necessary to obtain the result. _A fortiori_, the difficulty increases in the case of complex organisms, all of whose living elements cannot be attacked at the same moment by the destructive cause. A mechanical action, capable of destroying at one blow all the living parts of a complex being, of an animal, of a plant, must be of almost inconceivable power. The blow of a Nasmyth hammer would not be strong enough. The chemical alteration produced by a very toxic substance distributed throughout the blood, and thus brought into contact with each element, would produce a disorganization which, however rapid it were, could not be called instantaneous. And the same holds good of physical agents. But these are not the processes of nature under normal circumstances. They are accidents or devices. We shall leave on one side their consideration and we shall only deal here with the natural processes of the organism. Imagine it placed in a medium appropriate to its needs and following out without intervening complications the evolution assigned to it by its constitution. Experiment tells us that this natural evolution in every case known to us ends in death. Death supervenes sooner or later. For beings higher in organization, which we can bring into closer and closer resemblance to man, we find that they die of disease, by accident, or of old age. And as disease is an accident, we may naturally ask if what we call old age is not also a disease. However that may be, the mortal process, being never instantaneous, has a duration, a beginning, a development, an end—in a word, a history. It constitutes an intermediary phase between perfect life and certain death.