In the first place it does not follow that only _positive_ fluctuations of a factor, acting as a vital condition, result in _excitation_ in the existing vital processes. The _withdrawal_ of water produces a diametrically opposite effect. A muscle, from which water has been removed, if exposed to dry air or placed in a hypertonic salt solution, shows violent _excitation_, which manifests itself in great increase of irritability and development of fibrillary contractions. The breaking of a constant current which has for a long time flowed through a nerve or muscle also elicits a momentary excitation. Further, the abrupt removal of light may also bring about stimulation. To cite an example from the physiology of the single cell, I should like to call to your attention the interesting observations of _Engelmann_[17] on the _Bacterium photometricum_, of which he was the discoverer. When the field containing these organisms is suddenly darkened, all the individuals contained in the drop immediately dart forward for some distance, at the same time, as is usually the case, quickly rotating around their own axis, and then after a moment of immobility, swim on quickly in another direction. An analogous responsivity has also been shown by other single cell organisms, as has been pointed out by several observers and especially by _Jennings_.[18] In all these cases the excitation was produced by a lessening or total withdrawal of the factors which act as vital conditions; and even those who take the standpoint that only such factors are to be considered as stimuli which produce an _exciting_ effect, are compelled to regard these alterations as stimuli, in spite of the fact that they are _negative_ variations of external vital conditions. [17] _Th. W. Engelmann_: “Bacterium photometricum ein Beitrag zur vergleichenden Physiologie des Licht-und Farbensinns.” In Pflügers Archiv. Bd. 30. 1883. [18] _Jennings_: “Behavior of the lower organisms.” New York 1906.