The consequence is a reaction, in which the animal assumes _radiosa_ form. By the action of the stimulus a new state of the living substance is produced, and remains as long as the solution of caustic potash is contained in the medium. The solution of caustic potash is, therefore, a stimulus for the state of the vital system, which is manifested in the _limax_ form, whilst for the state of the system which shows itself in the _radiosa_ form, it is a vital condition. If I place the amœbæ of the _radiosa_ form once again in tap water, they assume the _proteus_ and then the _limax_ form. The withdrawal of the solution of caustic potash, the presence of which is a vital condition for the _radiosa_ state, acts as a stimulus, which results in a transition of the vital system to another state. By altering the medium I can at will bring about this change of form in the same individuals. In this way one and the same factor can figure as stimulus and vital condition, according to the state of the vital system on which it acts. Whilst its addition acts as stimulus in the one state, its withdrawal acts as a stimulus in the other state, which it has produced. The same fact is shown by the well-known example of _Artemia salina_, which on being placed in fresh water changes into _Branchipus stagnalis_ and, when again introduced into sea water, becomes once more _Artemia salina_. [16] _Max Verworn_: “Die polare Erregung der lebendigen Substanz durch den galvanischen Strom.” In Pflügers Archiv. f. d. ges. Physiologie Bd. 65, 1896. [Illustration: _A_ _B_ _C_ _D_ _E_ Fig. 1.] These facts show clearly that some stimuli can also be considered as vital conditions. In the absence of certain stimuli, life could not exist for any length of time. In the highly differentiated cell community of the animal organism, for instance, as a result of the coexistence of the cells and the tissues, many parts have forfeited in a measure their independence. An example of this is the skeletal muscle, which, in the absence of impulses from the nervous system, reaches a low level of chemical change and energy transformation. Here the nervous impulses which act as momentary stimuli, are also in the course of time indispensable vital conditions. Without them the muscle would gradually become atrophied from inactivity. The same applies 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?