This habit of the earthworm is the direct result of its reaction to light. It crawls away from ordinary light as bright as that of diffuse daylight, and, indeed, from light very much fainter than that of daylight. If, however, the light be decreased to a certain point, the worm will then turn and crawl toward the source of light. This lower limit has been found by Adams to be about that of .001 candle-metre. This corresponds to the amount of light of a dark night, and gives an explanation of why the worm leaves its burrow only at night, and also why it crawls back on the approach of dawn. It is also obvious that this response is useful to the animal, for if it left the burrow during the day, it would quickly fall a prey to birds. The blow-fly lays its eggs on decaying meat, on which the larvæ feed. The fly is drawn to the meat by its sense of smell, a simple and direct response to a chemical compound given off by the meat. The maggot that lives in the decaying meat is also attracted by the same odor, as Loeb has shown, and will not leave the meat, or even a spot on a piece of glass that has been smeared with the juice of the meat, so long as the odor remains. Here again the life of the race depends on the proper response to an external agent, and the case is all the more interesting, since the response of the fly to the meat is of no immediate use to the fly itself, but to the maggot that hatches from the egg of the fly. The movement toward or from a stimulating agent is, in some cases, brought about in the following way. Suppose an earthworm is lying in complete darkness, and light be thrown upon it from one side. The worm turns its head, as it thrusts it forward, to the side away from the light; and 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 contraction 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 reaction 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.