Life and Instinct 197 



night to feed. When stimulated by the light of returning dawn, 

 it retires again to its darkened subterranean chambers. Ex- 

 periment has shown that the worm responds to light of a certain 

 intensity, even crawling away from the light of a dull day, but 

 towards the much feebler light of an ordinary night. Such 

 movements, which were formerly assigned to instinct, are thus 

 seen to be prescribed by the direction and intensity of the rays 

 of light. That such responses are adaptive, or useful, is shown 

 by the fact that it is only "the early bird" that catches this 

 worm. 



The attraction of the candle for the moth and other night-fly- 

 ing insects is proverbial, and, as often happens, they will return 

 to it again and again until, singed in the flame, they fall to the 

 floor. In this case experiment has also proved x that the moth 

 always flies towards the source of light, and that it is more 

 sensitive at night. Moths pass the day in a kind of sleep, but 

 become very active at the approach of dusk. 



Earthworms, as well as many other animals, also creep into 

 the crevices of solid bodies, which for the time holds them like 

 a trap. They will crawl the whole length of a crevice before 

 leaving it, and the response in this case is not to light, but to 

 the mechanical friction of a solid body against their skins. 

 These are but a few of hundreds of illustrations which could 

 be given of the tropisms, or trial movements of an animal, 

 which are influenced by the direction and intensity of light or by 

 energy of some other form. While much still remains to be 

 explained, this is a step in advance of the former custom of 

 ascribing all such actions to "blind instinct." Changes in the 

 nervous system may determine the character of the response, 

 but the fact that the animal moves at all is due to an outside 

 cause. 



Physiology has also given us the term re-flex action, which 

 was first applied to vertebrates, and suggests a comparison be- 

 tween a mirror and the spinal cord. This was supposed to 

 receive sensory impressions from the surface of the body and to 



1 See especially the Studies in General Physiology of Professor Jacques 

 Loeb. Chicago, 1905. 



