1 84 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



animal kingdom, especially among insects, worms, etc. 

 This is called an instinct of self-preservation, and it 

 is assumed that the animal thus escapes from its 

 pursuers. The centre theory would assume a special 

 centre for this instinct. This is, however, only an- 

 other instance of a simple tropism. Many plants 

 and animals are forced to orient their bodies in a cer- 

 tain way toward solid bodies with which they come in 

 contact. I have given this kind of irritability the 

 name stereotropism. Like the positive and negative 

 heliotropism and geotropism, there is also a positive 

 and negative stereotropism, and there are also stereo- 

 tropic curvations. I have found, for instance, that 

 when a Tubularia is brought in contact with a solid 

 body, the polyp and the growing tip bend away. from 

 the body while the stolon sticks to it. The polyp 

 is negatively stereotropic and the stolon positively 

 stereotropic. Stereotropism plays a very important 

 part in the processes of pairing and the formation of 

 organs. The tendency of many animals to creep 

 into cracks and crevices has nothing to do with self- 

 concealment, but only with the necessity of bringing 

 the body on every side in contact with solid bodies. 

 I have proved this, for instance, in a peculiar species 

 of butterfly, Amphipyra, that is a fast runner. As 

 soon as free, it runs about until it finds a corner or a 

 crack into which it can creep. I placed some of these 

 animals in a box, one half of which was covered with 

 a non-transparent body, the other half with glass. I 

 covered the bottom of the box with small glass 



