Tropisms and Instincts as Adaptations 393 



shifted while the negative animal was in the middle of the 

 trough, it would turn and swim directly away, as before, from 

 the source of light. It could be kept in this negative state as 

 long as it did not come into contact with the ends. 



It appears that the positive condition in Cypridopsis is of 

 short duration, and ceases after a while either as a response 

 to contact or without any observable external factor causing 

 the change. 



This crustacean lives at the bottom of pools, amongst water- 

 plants, and here also, no doubt, the same change from one to 

 the other reaction takes place. What possible advantage it 

 may be to the animal to be kept continually changing in this 

 way is not at all obvious, nor, in fact, are we obliged to 

 assume that this reaction may be of any special use to it. 

 Indeed, it is far from obvious how the change that causes the 

 animal to swim toward the light when it is disturbed could 

 be of the least advantage to it. 



In another crustacean, one of the marine copepods, 

 Labidocera cestiva, it has been shown by Parker that the 

 male and female react in a somewhat different way both to 

 light and to gravity. The females are strongly negatively 

 geotropic, and this sends them up to the top of the water. 

 The males are very slightly negatively geotropic. The 

 females are strongly positively heliotropic toward light of 

 low intensity ; the males show the same response to a less 

 degree. To strong light the females are negative and the 

 males are indifferent. On the other hand, the males are 

 attracted to the females, probably in response to some 

 chemical substance diffusing from the females, since the 

 males show the same reaction when the females are en- 

 closed in an opaque tube through whose ends a diffusion 

 of substances may take place. This crustacean frequents 

 the surface of the ocean from sunset to sunrise. During the 

 day it retires to deeper water. Its migrations can be ex- 

 plained as follows : The females come to the surface at 



