194 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



expressed in the depth-migration of pelagic animals, is 

 frequently determined by the presence of substances 

 in the surfaces of the animal which are sensitive to 

 light. These substances, however, produce different 

 effects according to the intensity of the light or of 

 the temperature (or perhaps according to internal 

 conditions). They are further determined by the re- 

 lations of symmetry of the animals. The central 

 nervous system has nothing further to do with these 

 phenomena than that it furnishes the protoplasmic 

 connection between the skin and muscles. This dis- 

 agrees with the centre theory of these instincts, but 

 agrees with the segmental theory. 



7. One might think that these ideas held good only 

 for Invertebrates. Goltz has, however, made a re- 

 markable discovery which seems to confirm the opin- 

 ion that in Vertebrates the conditions are practically 

 the same. A female dog that has given birth to a 

 young one bites off the navel cord, licks the young, is 

 very affectionate towards it, and allows no stranger to 

 touch it. These motherly instincts are inherited, and 

 there is no doubt that with the act of giving birth and 

 the resulting processes in the sexual organs changes 

 take place in the animal which make these instincts 

 possible. One might think, especially in this case, of 

 centres in the central nervous system which are stim- 

 ulated directly through the nerves of the uterus. Now 

 Goltz found that these instincts are also fully devel- 

 oped in dogs whose spinal cord is severed so far up 

 that the stimuli from the uterus cannot reach the 



