The philosophy of biology . called heliotropic, and those of its roots in theperpendicular direction are called geotropic. Themotion of the freely-moving larva of a barnacle, forinstance, in swimming towards a source of light arecalled phototactic. In all these cases we have to think of the stimulusas a field of energy in the sense in which physicistsspeak of electric, or magnetic, or electromagnetic, or 1 Many of Jacques Loebs remarkable investigations point in this direction. THE VITAL IMPETUS 145 thermal, or gravity fields. In all these cases thefactors affecting the movements of the organi
The philosophy of biology . called heliotropic, and those of its roots in theperpendicular direction are called geotropic. Themotion of the freely-moving larva of a barnacle, forinstance, in swimming towards a source of light arecalled phototactic. In all these cases we have to think of the stimulusas a field of energy in the sense in which physicistsspeak of electric, or magnetic, or electromagnetic, or 1 Many of Jacques Loebs remarkable investigations point in this direction. THE VITAL IMPETUS 145 thermal, or gravity fields. In all these cases thefactors affecting the movements of the organism aredirected ones. An electric field, for instance, (i), is produced byplacing the electrodes of a galvanic cell at oppositeextremities of a water-trough : we imagine the electronsmoving from one side of the trough to the other inparallel lines, and in a certain direction. A lightfield (2) would be produced by the radiation of lighttravelling in straight lines through the water. The movements of the organism displaying a. (2) FIG. 18. tropism or a taxis are not caused by the stimuli of thefield, but are only directed by it. In the absence ofthese stimuli it would swim at random. In a field,however, it will orientate itself in some direction withreference to the lines of force. A positively photo-tactic animal swims towards the focus from whichthe light radiation emanates, and a negativelyphototactic one swims in the other direction. Onthe theory of tropistic and tactic movements thisorientation is produced by the differential stimulationof the opposite sides of the organism. Let us take asa concrete example the case of a caterpillar whichcreeps up the stem of a plant to feed on the tendershoots near the apex. The animal possesses an 146 THE PHILOSOPHY OF BIOLOGY elongated body, with muscles beneath the integument,and sensory nerve-endings in the latter. Its musclesare in a state of tone, that is, they are normallyalways slightly tense. The incident rays of lightaffect the der
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