THEORY OF INSTINCTS 193 



once more be<jin their downward migration. But the 



o o 



pelagic animals also show another depth-movement 

 of a greater period, which corresponds more nearly 

 with the migration of birds of passage. Chun has 

 found that in the Bay of Naples during summer cer- 

 tain forms also remain at a greater depth during the 

 night, never coming to the surface. This is probably 

 due to the higher temperature which the surface of 

 the water has in summer. I have found that certain 

 animals, for instance, the larvae of Polygordius, are 

 positively heliotropic in a low temperature, while 

 in a higher temperature they become negatively 

 heliotropic (4). 



I have also mentioned that geotropism also plays a 

 part in these depth-migrations. The same circum- 

 stances which make the animals negatively helio- 

 tropic also make them positively geotropic, and vice 

 versa. Thus I was able to show that in a low tem- 

 perature the larvae of Polygordius are also negatively 

 geotropic, \vhile in a high temperature they are posi- 

 tively geotropic (4). By means of this geotropism 

 they are also forced in the dark to go to the surface 

 when the temperature of the water is low. It is also 

 probable that in many forms internal conditions simi- 

 lar to the nyctitropic phenomena in plants are in- 

 fluential in causing periodic depth-migrations. 1 We 

 thus find that the migratory instinct, as far as it is 



1 This may account for the periodic migrations of certain animals (Medusa?) 

 in polar regions. In such animals, changes in the specific gravity may take 

 the place of heliotropic reactions. 



