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 larvze 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 vzce 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 
