THE EMERGENCE OF VISION 



103 



creatures depending essentially in their behaviour on non-visual 

 stimuli. 



For example, the scallop has numerous visual cells around the edge of its 

 mantle, and if these are stimulated by the " sight " of its enemy, the starfish, 

 no response except the awareness of the presence of something is elicited, and 

 no attempt at flight is made ; but whenever some extract of starfish is added 

 to the water in which the animal lies, the scallop immediately runs away (Dakin, 

 1909 ; von Uexkiill, 1921). ]\Ioreover, in Pecten, no response is called forth 

 until the object moves, and any movement of any object excites the same 

 response, a protrusion of the tentacles ; these are endowed with organs of 



Fig. 74. — The Sensory Reactions of the Water Beetle. 



A watery meat extract is contained in tiie bag. The feeding responses of 

 Dytiscus marginalis show its dependence on chemical stimuli rather than vi.sual 

 (Tinbergen, Study of Instinct ; Clarendon Press). 



chemical and tactile sensitivity which exjilore the object " intelligently", and 

 on the results of their findings the animal either eats or flees (Dakin, 1910). 

 The purpose of this response is obviously to secure further information in a form 

 in which it is analysable. Even in man the olfactory sense organs are relatively 

 more fully developed than the visual at birth ; a fish with its olfactory nerves 

 severed ceases to feed spontaneously (Steiner, 1888) ; and the lately-born rabbit 

 will die of starvation if deprived of the sense of smell because it cannot find the 

 teats of its mother, even although it has been allowed to make use of its eyes 

 before it has suffired the loss of the more fundamental sense. 



