In considering this question, one point must not be omitted. This is the fact that one of the chief characteristics of the vital process is, that it undergoes continuous change. A vital process involves not simply an alteration in metabolism or transformation of energy in the sense that the same chemical processes continuously reoccur in the same manner. Such a view could only be admissible for the observation of living substance during a limited period. An investigation over a long period of time shows rather that every living system alters as long as it exists, although this alteration is very gradual. The constituent processes, in short, continuously undergo metabolic change both quantitative and qualitative in nature. If we observe the occurrences in a living system at various moments of the cycle of life, we will find that the condition differs qualitatively at each period. The progressive alteration of the system is such that every state of living substance conditions another, by which it is followed. No state can permanently exist as such. Every state is the product of the preceding, as it in turn conditions its successor. Consequently the relations of the system to the surrounding medium also undergo alteration, even when the external factors themselves in no way alter. That which today is still a vital condition, is not in consequence necessarily one tomorrow. These progressive changes exist continuously until the death of the system takes place. They characterize life. It is development, and life cannot exist without development. Death is only the last phase of development. The individual constituent processes of metabolism gradually change to such a degree that they can no longer work harmoniously together. Then the chain of processes is interrupted at one point or another. The system develops into death or, on the other hand--and this, as _Weissman_ especially emphasizes, is realized in the case of unicellular organisms--a corrective process takes place, a process of cell division by which the original state of the cell is restored and development begins anew and in a similar manner.