394 Evolution and Adaptation 
night, because they are positively heliotropic to weak light, 
and also because they are negatively geotropic. They go 
down during the day, because they react to bright light 
more strongly than to gravity. The males follow the fe- 
males, largely because they react positively chemotactically 
toward the females. 
Some other animals respond in a somewhat different way 
to light, as shown by the fresh-water planarians. These 
animals remain during the day under stones, where the 
amount of light is relatively less than outside. If they are 
placed in a dish in the light in front of a window, they crawl 
away from the light, but when they reach the back of the 
dish they do not come to rest, but continue to crawl around 
the sides of the dish even toward the light. The light 
makes the worms restless, and while they show a negative 
response as long as they are perfectly free to move away 
from the light, they will not come to rest when they come 
to the back of the dish if they are there still in the light, 
" because the irritating action of the light on them is stronger 
than its directive action. If, however, in crawling about 
they come accidentally into a place less bright than that in 
which they have been, they stop, and will not leave this 
somewhat darker spot for a brighter one, although they 
might leave the newly found spot for one still less bright. 
At night the planarians come out and wander around, 
which increases their chance of finding food, although it 
would not be strictly correct to say that they come out in 
search of food. If, however, food is placed near them, a 
piece of a worm, for example, they will turn toward it, 
being directed apparently by a sense of smell, or rather of 
taste. 
The heliotropic responses of the planarians appear to be of 
use to them, causing them to hide away in the daytime, and 
to come out only after dark, when their motions will not dis- 
cover them to possible enemies. But some of the planarians 
