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 e@stiva, 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 
