The very close connection between the reactions of this caterpillar and its mode of life is perfectly obvious. The entire series of changes seems to have for its “purpose” the survival of the individual by bringing it to the place where it will find its food. It may seem natural to conclude that these responses have been acquired for this very purpose, but let us not too quickly jump at this obvious conclusion until the whole subject has been more fully examined. The upward and downward movements of some pelagic animals have been shown to depend on certain tropic responses. Every student of marine zoology is familiar with the fact that many animals come to the surface at night, and go down at the approach of daylight. It has been shown that this migration is due largely to a response to light. Light can penetrate to only about four hundred metres in sea-water, and there is complete darkness below this level. It has been shown that the swimming larvæ of one of the barnacles is positively heliotropic in a weak light, but negatively heliotropic in a stronger light. Animals having responses like these will come to the surface as the light fades away in the evening and remain there until the light becomes too bright in the following morning. They will then become negatively heliotropic and begin to go down. When they reach a level where the intensity of the light is such that they become positively heliotropic, they will turn and start upward again. Thus during the day they will keep below the surface, remaining in the region where they change from positive to negative, and _vice versa_. It would not be difficult to imagine that this upward and downward migration of pelagic animals is useful to them, but, on the other hand, it may be equally well imagined that the response may be injurious to them. Thus it might be supposed that certain forms could procure their food by coming to the surface at night, and avoid their enemies by going down during the day. But it is difficult to see why organisms that serve as prey should not have acquired exactly the opposite tropisms in order to escape. Some of these marine forms are also geotropic. Loeb has determined that “the same circumstances that make the animals negatively heliotropic also make them positively geotropic, and _vice versa_.” It was found, for instance, that the larva of the marine worm Polygordius is negatively geotropic at a low temperature, while at a higher temperature it is positively geotropic. This response would drive the animals upward when the water becomes too cold, and back again if the surface water becomes too warm; but whether the response is so adjusted that the animals keep, as far as possible, in water of that temperature that is best for their development, we do not know. We can easily imagine that within wide limits this is the case.