Physiology of Central Nkuvous System 09 



certain group of reflexes — the so-ciillecl iii.stinct.s. InstiiicU 

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

 is phrased the meaning seems to be that they arc inherited 

 reflexes so purposeful and so compHcated in character that 

 nothing short of intelligence and experience could have j)r()duce(l 

 them. To this class of reflexes belongs the habit j)()--e-.-ed 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 e^gs 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 in the ganglion-cells ? How can we 

 explain the inheritance of such instincts 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 ])ossible. 

 Among the elements which compose these comj^licated instincts, 

 the tropisms (heliotropism, chemotropism, geotro])ism, stere- 

 otropism) play an important part. These tropisms are identical 

 for animals and plants. The explanation of them depends tirst 

 upon the specific irritability of certain elements of the body- 

 surface, and, second, upon the relations of sx-mmctry of the 

 body. Symmetrical elements at the surface of the body have 

 the same irritability; uns>Tnmetrical elements have a different 

 irritability. Those nearer the oral i^ole 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 stinui- 

 lation in such a way that symuK^trical 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 



