Tropisms and Instincts as Adaptations 383 



of its hole, and lies stretched out on the surface of the ground. 

 It procures its food at this time, and the union of the individ- 

 uals takes place. In the early morning the worm retires into 

 its burrow. 



This habit of the earthworm is the direct result of its re- 

 action to light. It crawls away from ordinary light as bright 

 as that of diffuse daylight, and, indeed, from light very 

 much fainter than that of daylight. If, however, the light 

 be decreased to a certain point, the worm will then turn and 

 crawl toward the source of light. This lower limit has been 

 found by Adams to be about that of .001 candle-metre. This 

 corresponds to the amount of light of a dark night, and gives 

 an explanation of why the worm leaves its burrow only at 

 night, and also why it crawls back on the approach of dawn. 

 It is also obvious that this response is useful to the animal, 

 for if it left the burrow during the day, it would quickly fall 

 a prey to birds. 



The blow-fly lays its eggs on decaying meat, on which the 

 larvae feed. The fly is drawn to the meat by its sense of 

 smell, a simple and direct response to a chemical compound 

 given off by the meat. The maggot that lives in the decay- 

 ing meat is also attracted by the same odor, as Loeb has 

 shown, and will not leave the meat, or even a spot on a 

 piece of glass that has been smeared with the juice of the 

 meat, so long as the odor remains. Here again the life of 

 the race depends on the proper response to an external 

 agent, and the case is all the more interesting, since the 

 response of the fly to the meat is of no immediate use to 

 the fly itself, but to the maggot that hatches from the egg 

 of the fly. 



The movement toward or from a stimulating agent is, in 

 some cases, brought about in the following way. Suppose 

 an earthworm is lying in complete darkness, and light be 

 thrown upon it from one side. The worm turns its head, 

 as it thrusts it forward, to the side away from the light ; and 



