392 Evolution and Adaptation 
to the surface at night, and avoid their enemies by going 
down during the day. But it is difficult to see why organ- 
isms that serve as prey should not have acquired exactly the 
opposite tropisms in order to escape. 
Some of these marine forms are also geotropic. Loeb has 
determined that “the same circumstances that make the 
animals negatively heliotropic also make them positively 
geotropic, and vice versa.” It was found, for instance, that 
the larva of the marine worm Polygordius is negatively 
geotropic at a low temperature, while at a higher tempera- 
ture it is positively geotropic. This response would drive 
the animals upward when the water becomes too cold, and 
back again if the surface water becomes too warm; but 
whether the response is so adjusted that the animals keep, as 
far as possible, in water of that temperature that is best for 
their development, we do not know. We can easily imagine 
that within wide limits this is the case. 
The change from positive to negative can also be brought 
about in other ways. One of the most striking cases of 
this sort is that described by Towle in one of the small 
crustaceans, Cypridopsis vidua. It was found that after an 
animal had been picked up in a pipette its response was al- 
ways positive; that is, it swam toward the light, no matter 
what its previous condition had been. The disturbance 
caused by picking the animal up induced always a positive 
response towards light. If the light were moved, the Cyp- 
ridopsis followed the light. In this way it could be kept 
positive for some time, but if it came to rest, or if it came into 
contact with the sides or end of the trough, it became, after 
a short time, negatively heliotropic, and remained negative as 
long as it could be kept in motion, without being disturbed, or 
coming into contact with a solid object. If when positive 
it were allowed to reach the glass at the end of the trough, it 
would swim about there, knocking against the glass, and then 
soon turn and swim away from the light. If the light were 
