CHAPTER XII 



SENSES, INSTINCTS, AND INTELLIGENCE 



§ I. The Eye. § 2. The Ear. § 3. The other Sense-Organs. 4. The 

 Inclined Plane of Animal Behaviour. § 5. Instinctive Behaviour. 

 § 6. Intelligent Behaviour. 



It is convenient to consider the senses along with the 

 behaviour of the bird. All Uving means activity, and part 

 of this is agency, when the organism acts on its environment. 

 If this acting is to be effective, the creature must be aware 

 of its environment ; and the sense-organs are the " recep- 

 tors " of the stimuli which form the raw materials or data 

 of the awareness. They are the gateways of knowledge. 

 The Golden Eagle must have a very precise estimate of 

 the position of the grouse if it is to clutch it in its talons 

 without ceasing to fly, and the Osprey must have an even 

 more difficult problem with the swimming fish. 



Even when there is no emphatic agency, the sense 

 organs are indispensable in the adjustment of the organism 

 to its environment. They are needed for reactions as well 

 as for actions. The brooding bird, oppressed by the heat 

 of the sun, will turn in its nest so that it secures some relief. 

 In cold weather the internal production of heat is auto- 

 matically increased so as to compensate for the loss of heat 

 into the frosty air. In both cases the adjusting reactions 

 follow the stimulation of sensitive nerve-endings in the 

 skin. We see, then, that through its sensory system the bird 

 actively tests the outer world and is also more passively 

 recipient of surrounding influences. 



How many different kinds of sensory " receptors " 

 there are it is difficult to say, for besides eyes, ears, olfactory 



318 



