The movements of the organism displaying a tropism or a taxis are not caused by the stimuli of the field, but are only directed by it. In the absence of these stimuli it would swim at random. In a field, however, it will orientate itself in some direction with reference to the lines of force. A “positively phototactic” animal swims towards the focus from which the light radiation emanates, and a “negatively phototactic” one swims in the other direction. On the theory of tropistic and tactic movements this orientation is produced by the differential stimulation of the opposite sides of the organism. Let us take as a concrete example the case of a caterpillar which creeps up the stem of a plant to feed on the tender shoots near the apex. The animal possesses an elongated body, with muscles beneath the integument, and sensory nerve-endings in the latter. Its muscles are in a state of “tone,” that is, they are normally always slightly tense. The incident rays of light affect the dermal sense-organs, stimulating ganglionic centres and setting up efferent impulses which descend to the muscles. Let us suppose the animal is moving so that the longitudinal axis of its body is at an angle, say of 45°, to the direction of the incident light: one side of the body is therefore stimulated and the other is not. The stimulation of the lighted side sets up efferent nerve impulses which descend to the muscles of this side and increase their tone (or else the lack of stimulation of the other side produces impulses which inhibit the muscular tone, or impulses which would otherwise preserve the tone cease in the absence of light stimulation). In any case the muscles of the lighted side contract, and the body of the caterpillar moves so that it sets itself parallel to the direction of the radiation. Both sides of the body are then equally stimulated and the animal moves towards the light. The animal feeds and it then creeps back down the plant. Why does it do this? Because, says Loeb, the act of feeding has reserved the “sign” of the taxis. Before, when it was hungry, it was positively phototactic, but the act of feeding (all at once, it would appear, before digestion and assimilation of the food itself) has produced chemical substances in the muscles which cause the latter to relax in response to an impulse which previously produced contraction. The nervous link is not, of course, a necessary one. The stimulation by the energy of the field may affect the muscle substance directly, or it may, as in the case of a protozoan animal, affect the general body protoplasm in the same way. In the majority of cases, however, the orientation would be affected through the chain of sense-organ, afferent nerve, nerve centre, efferent nerve, and effector organ. This is the chain of events which on this hypothesis causes a moth to fly into a flame, or a sea-bird to dash itself against the lantern of a lighthouse.