=Behavior of Many Animals Often an Automatic Adjustment to Simple External Agents.--=It is a far cry, of course, from the responses of such a machine as a windmill to the responses of even the simplest living thing, but in spite of the broad gap between the two, there is much reason to believe that the behavior of many living organisms is due in a marked degree to the directive effects of comparatively simple external factors rather than to the complex internal volitions the casual observer is likely to attribute to them. =Tropisms.--=It is a marked characteristic of all living protoplasm that it has the power of responding to external stimuli. This power of response is termed _excitability_ or _irritability_. In describing the motor responses of living organisms to stimuli resulting from a change in surroundings the term _tropism_ (Gr. _Trope_, turning) is frequently used and the kind of stimulus is indicated by a prefix. Thus the term phototropism means a turning or _orientation_ brought about by means of light. An organism which reacts by a movement toward the source of light is said to be _positively phototropic_, one which moves away from it, _negatively phototropic_. By using such a neutral terminology the physiologist avoids implying that necessarily "likes" or "dislikes" or any other psychic reaction enter into the movements. Several kinds of tropisms are recognized, such as _phototropism_ or _heliotropism_, reaction to light; _thermotropism_, reaction to heat; _electrotropism_ or _galvanotropism_, to electric current; _geotropism_, to gravity; _chemotropism_, to a chemical; _rheotropism_, to current; _thigmotropism_ or _stereotropism_, to contact; and _chromotropism_, to color. =Many Animals Show Tropic Responses.--=Many of the lower animals seem to have their movements determined more or less mechanically by the action of such external factors, some being positively, others negatively responsive to a given kind of stimulus, or the same individual may be at one time positive, at another negative, according to modifying conditions to be mentioned presently.