Tropisms and Instincts as Adaptations 391 



degrees C, and above this temperature point they become 

 restless and wander about. 



The very close connection between the reactions of this 

 caterpillar and its mode of life is perfectly obvious. The 

 entire series of changes seems to have for its " purpose " the 

 survival of the individual by bringing it to the place where 

 it will find its food. It may seem natural to conclude that 

 these responses have been acquired for this very purpose, 

 but let us not too quickly jump at this obvious conclusion 

 until the whole subject has been more fully examined. 



The upward and downward movements of some pelagic 

 animals have been shown to depend on certain tropic re- 

 sponses. Every student of marine zoology is familiar with 

 the fact that many animals come to the surface at night, 

 and go down at the approach of daylight. It has been 

 shown that this migration is due largely to a response to 

 light. Light can penetrate to only about four hundred 

 metres in sea-water, and there is complete darkness below 

 this level. It has been shown that the swimming larvae of 

 one of the barnacles is positively heliotropic in a weak light, 

 but negatively heliotropic in a stronger light. Animals 

 having responses like these will come to the surface as the 

 light fades away in the evening and remain there until the 

 light becomes too bright in the following morning. They 

 will then become negatively heliotropic and begin to go 

 down. When they reach a level where the intensity of the 

 light is such that they become positively heliotropic, they 

 will turn and start upward again. Thus during the day 

 they will keep below the surface, remaining in the region 

 where they change from positive to negative, and vice versa. 



It would not be difficult to imagine that this upward and 

 downward migration of pelagic animals is useful to them, but, 

 on the other hand, it may be equally well imagined that the 

 response may be injurious to them. Thus it might be sup- 

 posed that certain forms could procure their food by coming 



