muscles — in all but the simplest animals. But they resemble the plant tro- 

 pisms in that they take place automatically, or mechanically. That is, they 

 are in no way voluntary, or controlled by a "will". From the fact that the 

 moth flies to its own destruction we may at least argue that there is no 

 intention in the act. Although the animal has a very good set of compound 

 eyes and a comparatively complex nervous system, it seems to have no 

 choice. 



Responses to Gravity Whatever "gravity" is, it acts upon animals and 

 plants, as well as upon stones and planets. Some animals adjust themselves 

 to the action of gravity in a variety of ways that are tropic, but not all. The 

 housefly, for example, seems to be indifferent to the direction of gravity; it 

 will crawl upon a surface in any plane and in any direction, and it will 

 come to rest in any possible position. Yet, if you place a fly on its back, it 

 will right itself, as would a backboned animal or a starfish. 



Many adult insects, when they alight on a tree, assume a position with 

 the head pointing upward; other kinds always rest with the head pointing 

 downward. In still other species, the source of light determines the position, 

 rather than gravity. In some species the young larva crawls toward the tip 

 of the twig. This movement is adaptive since it brings the young insect to 

 its food. But in some species the animal moves toward the light, whereas in 

 others it moves up, as we can tell by experimenting. 



Some of the simple marine animals appear to be influenced by both light 

 and gravity. Certain species of minute crustaceans swim near the surface 

 only at night; under the influence of light they become negatively geotropic 

 — that is, they swim down from the surface. Experienced fishermen have 

 learned that many species of fish are to be found at varying depths accord- 

 ing to the time of day: in a given lake, however, at a given hour, hundreds 

 of fish of the same kind will be found at about the same level. There is 

 probably a combination of influences at work — temperature, as well as light 

 and gravity. And many of the variations in an animal's reactions appear to 

 result from changes in the chemical or physical state. The larvae moving 

 toward the tip of the twig show a reversed tropism after they have eaten. 



Reflex Action When you are tickled, you draw away the touched 

 part. When something approaches your eye, you wink. A slight touch in- 

 side the nose leads to a sneeze. A solid particle on the lining of the windpipe 

 makes you cough. When you place a solid in a baby's palm the hand closes 

 down. Such reactions to particular stimulations have always been known. 

 In the members of any species of animal they are remarkably uniform. And 

 in any individual they are remarkably constant. 



A famous French philosopher and mathematician, Rene Descartes (1596- 

 1650), suggested for this type of action in animals the name reflex. It is as 

 if a force entering the body at some point — the skin or the eye, for example 



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