384 Evolution and Adaptation 
as it again moves forward, it continues to bend its head 
away from the light, until it is crawling directly away from 
the source. When the light first strikes the worm, the two 
sides will be differently illuminated. This causes a bending 
of the head, as it stretches forward, toward the side of less 
illumination, and the bending is due to a stronger contrac- 
tion of some of the muscles on the less illuminated side; 
at least the reaction appears to be due to a simple response 
of this kind. When the body has been so far turned that 
the two sides are equally illuminated, the muscles of the two 
sides will contract equally, and the movement will be straight 
forward and away from the light. If the reaction is as simple 
as this (which is in principle the explanation advanced by 
Loeb), the result is a simple reflex act, and need not involve 
any consciousness or intentional action on the part of the 
worm to crawl away from the light. In fact, the same reac- 
tion takes place when the brain is removed, not so quickly or 
definitely, it is true, but this may be due to the removal of 
the anterior segments of the worm, in which part the skin 
appears to be more sensitive to light than elsewhere. 
Another factor that plays an important réle in the habits 
of the earthworm is the response to contact, — the so-called 
stereotropism. If, in crawling over a flat surface, the worm 
comes in contact with a crevice, it will crawl along it, and 
refuse to leave until the end is reached. The contact holds 
the worm as strongly as though it were actually pulled into 
the crevice. It can be forced to leave a crevice only by 
strong sunlight, and then it does not do so at once. If the 
worm crawls into a small glass tube, it is also held there 
by its response to contact, and the smaller the tube, the more 
difficult is it to make the worm leave by throwing strong sun- 
light upon it. 
Loeb has found that when winged aphids, the sexual 
forms, are collected in a tube, and the tube is kept in a 
room, the aphids crawl toward the light. This happens in 
