1 9 o COMPARATIVE PHYSIOLOGY OF THE BRAIN 



in the skin of the animal which are sensitive to light, 

 upon which substances the heliotropism depends, or 

 that through the consumption of food the action of 

 these substances is indirectly prevented. 



6. The analysis of other instincts, for instance, the 

 migratory instinct of animals, leads to the same result 

 as the analysis of the protective instincts. These in- 

 stincts are not functions of certain localised " centres," 

 but of irritabilities of certain peripheral structures and 

 of the connection of the same with the muscles, whereby 

 the central nervous system only serves as a protoplas- 

 mic connection. It would naturally be more inter- 

 esting to select for our discussion the migrations of 

 birds, but it is difficult to make laboratory experi- 

 ments on this subject, and without laboratory experi- 

 ments we cannot easily obtain reliable results. For 

 this reason I have made use of another class of 

 periodic migrations, namely, the periodic depth-migra- 

 tions of pelagic animals. A great number of these 

 animals begin a vertical upward migration toward the 

 surface of the ocean in the evening, while in the 

 morning they migrate downwards. It is a remarkable 

 fact that these forms never go below a depth of four 

 hundred metres in their downward migrations. This 

 fact suggests that the light is the controlling power 

 in these depth-migrations. Water absorbs the light, 

 and the thicker the layer of water the more the light 

 is absorbed. It has been found that at a depth of 

 four hundred metres a photographic plate is no longer 

 affected. My investigations show that the movable 



