196 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



intimately connected with the sexual development. I 

 have never found true heliotropism in the workers, 

 while the sexually mature males and females are de- 

 cidedly positively heliotropic. Wherever these tran- 

 sitory changes of irritability are present, it requires 

 experimental work to succeed in the analysis of the 

 instinct. 



A second series of difficulties arises from the influ- 

 ence of associative memory in many cases of instincts. 

 The periodic depth-migration of marine animals is a 

 simple case of instinctive migrations, while the migra- 

 tions of birds or the accomplishments of the carrier- 

 pigeon seem to be complicated by memory. It seems 

 to be certain that the carrier-pigeon finds its way back 

 by its visual memory of the locality from which it 

 started. In the same way the migration of birds may 

 be determined, if it is true that migrating birds return 

 to their old nest. In the case of the birds, there is 

 present in addition a purely inherited, instinctive 

 element which causes restlessness at the time of 

 migration. This restlessness and, perhaps to a cer- 

 tain extent, the direction of its flight are susceptible 

 of a purely physiological analysis. The element of 

 memory complicates many instinctive actions of wasps. 

 I have had a chance to observe solitary wasps and am 

 convinced that they find the way to their nest by means 

 of the visual memory of the locality where it is situ- 

 ated. The same is apparently true of bees and pos- 

 sibly of ants. (See Chapter XV.) 



9. The analysis of instincts from a purely physio- 



