70 Instinct and Intelligence 



sensory organs have been evolved by natural 

 modes of energy acting on differentiated 

 living matter, but they also illustrate the 

 gradual loss of these organs when deprived of 

 their appropriate modes of stimulation. For 

 instance, some aquatic worms have visual 

 organs, but in the common earth-worm, living 

 as it does without sunlight these organs have 

 disappeared. But although these animals do 

 not possess eyes, when the anterior ends of 

 their bodies are exposed to a stream of bright 

 light the worm speedily retreats into its burrow, 

 while if other parts of the animal's body are 

 alone illuminated it remains perfectly passive. 

 It would seem that the energy attached to 

 waves of light affects the ending of cutaneous 

 nerves of the animal's head, and reaches its 

 cerebral ganglia, thus setting free some of their 

 latent purposive energy, which becomes mani- 

 fest in the co-ordinate action of certain groups 

 of muscles, and so to instinctive movements of 

 the animal's body. Some fishes and reptiles 

 are influenced by light which falls on the skin 

 alone; a frog whose eyes have been removed 

 will, if light of suitable intensity be allowed to 



