THEORY OF INSTINCTS 195 



brain (6). It is probable that certain substances 

 which are developed during the pregnancy, birth, and 

 lactation influence the character of the animal, just 

 as certain poisons, for instance, alcohol, tobacco, or 

 morphine, influence the reactions of a human being. 

 It is of course possible that the sympathetic plays a 

 part here, although this has been rendered improbable 

 through the more recent experiments of Goltz and 

 Ewald and of Ribbert 1 



8. We have confined our attention to the simplest 

 instincts, for these are best adapted for a complete an- 

 alysis. Should we attempt a complete enumeration 

 and discussion of instincts, we should have to devote 

 several volumes to that subject alone. We should 

 like to call attention to the conditions which are re- 

 sponsible for the fact that many instincts are difficult 

 to analyse. One source of complication lies in the 

 fact already mentioned, that changes in the condition 

 of the blood, for example, those produced by metabol- 

 ism, may change the forms of irritability and reaction. 

 The young caterpillar of Porthesia is only heliotropic 

 so long as it is starving, while it becomes indifferent 

 to light as soon as it is fed. In plant-lice, the helio- 

 tropic irritability is connected with the growth of 

 wings. The wingless forms may or may not show 

 positive heliotropism ; if we produce wings (by 

 lowering the temperature or by letting the plant on 

 which it lives dry out), the animal becomes energetic- 

 ally positively heliotropic. In ants heliotropism is more 



1 See next chapter. 



