Physiology of Central Nervous System 69 



certain group of reflexes — the so-called instincts. Instincts 

 are defined in various ways, but no matter how the definition 

 is phrased the meaning seems to be that they are inherited 

 reflexes so purposeful and so complicated in character that 

 nothing short of intelligence and experience could have produced 

 them. To this class of reflexes belongs the habit possessed by 

 certain insects of laying their eggs on the material which the 

 larvae will afterward require for food. When we consider that 

 the female fly pays no attention to her eggs after laying them, 

 we cannot cease to wonder at the seeming care which nature 

 takes for the preservation of the species. How can the action 

 of such an insect be determined if not by mysterious structures 

 which can only be contained ia the ganglion-cells ? How can we 

 explain the inheritance of such instiucts if we believe it to be a 

 fact that the ganglion-cells are only the conductors of stimuli ? 

 It was impossible either to develop a mechanics of instincts or 

 to explain their inheritance in a simple way from the old stand- 

 point, but our conception makes an explanation possible. 

 Among the elements which compose these complicated instincts, 

 the tropisms (heliotropism, chemotropism, geotropism, stere- 

 otropism) play an important part. These tropisms are identical 

 for animals and plants. The explanation of them depends first 

 upon the specific irritability of certain elements of the body- 

 surface, and, second, upon the relations of sjonmetry of the 

 body. Symmetrical elements at the surface of the body have 

 the same irritability; unsymmetrical elements have a different 

 irritability. Those nearer the oral pole possess an irritability 

 greater than that of those near the aboral pole. These circum- 

 stances force an animal to orient itself toward a source of stimu- 

 lation in such a way that symmetrical points on the surface 

 of the body are stimulated equally. In this way the animals 

 are led without will of their own either toward the source of 

 the stimulus or away from it. Thus there remains nothing 

 for the ganglion-cell to do but to conduct the stimulus, and 



