146 THE PHILOSOPHY OF BIOLOGY 



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 photo- 

 tactic, 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 proto- 

 plasm in the same way. In the majority of cases, 



