39 2 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 



