THE NATURE OF STIMULATION 35 



to all other tissues of our bodies. The functional stimuli are for 

 them at the same time vital conditions. These vital conditions 

 undergo fluctuations and interruptions but at each alteration 

 from a given state they act as stimuli. 



Stimulus is every change in the vital conditions. But is this 

 definition complete? Are we really justified in regarding every 

 alteration in the vital conditions as a stimulus? 



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 con- 

 tinuously 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 pro- 

 cesses, 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 

 diflfers qualitatively at each period. The progressive alteration 

 of the system is such that every state of living substance condi- 

 tions another, by which it is followed. No state can perma- 

 nently exist as such. Every state is the product of the pre- 

 ceding, 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 metab- 

 olism 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 



