THEOR Y OF INS TINC TS 183 



more probable that they are determined by properties 

 which are common to animals and plants. From what 

 has been said above it is easy to infer what these 

 properties are : First, heliotropic animals as well as 

 heliotropic plants must contain a substance on their 

 surfaces which undergoes a chemical change when 

 subjected to the influence of the light, and this change 

 must be able to produce changes of tension in the 

 contractile tissue. Second, heliotropic animals as 

 well as heliotropic plants possess symmetry of form 

 and a corresponding distribution of the irritabilities. 

 These two groups of conditions determine the helio- 

 tropic reaction unequivocally. But what has the 

 central nervous system to do with this " instinct " of 

 the moth to fly into the flame, or, as we may now 

 say, with its heliotropism ? I believe nothing more 

 than that the nervous system contains a series of 

 segmental ganglia which establish the protoplasmic 

 connection between the skin and muscles. If we 

 destroy the central nervous system, the heliotropic 

 reactions in many animals cease, but mainly for the 

 reason that the connection between the skin, or the 

 eyes, which are affected by the light, and the muscles, 

 is interrupted. Hence it would be just as wrong to 

 assume a specific centre for the flight of the moth 

 into the flame as it would to assume a specific centre 

 for the going of Palaemonetes to the anode. 



3. We will select another instinct, namely, the 

 habit many animals have of crawling into cracks and 

 crevices. This " instinct " is very prevalent in the 



