INSTINCTS 245 



that smell like meat or like manure are meat and manure ; and 

 if the eggs are deposited in such materials, the young are supplied 

 with food. Therefore these instincts are, on the whole, beneficial, 

 or at least not fatal, to the species. 



293. Instincts may be modified. We should expect that 

 eating instincts would be so well fixed in the organization of 

 an animal that nothing could change them. But although we 

 cannot teach a frog to eat food that is at rest, or to avoid use- 

 less bits of dangling bait, it is possible to modify the instincts 

 of other animals in various ways. The dog who will refrain 

 from eating when he is not hungry illustrates the modification 

 of reflexes by the physiological state of the body. That is, 

 when the animal is no longer hungry, the condition of the 

 blood and of the other juices is different from what it is in 

 a hungry dog ; and the " hungry " nerves and muscles behave 

 one way in the presence of food, whereas the sated organism 

 behaves in a different way. A different situation is presented 

 by the goose that has had her food stuffed down her throat by 

 hand for some time. After a while the animal is no longer 

 stimulated by the sight of grain etc. to open her beak and bend 

 her neck and take up the food. We might say that for lack of 

 exercise the instinct has disappeared. It is probable, however, 

 that only certain of the reflexes have dropped out of the chain. 



294. How an organism learns. In an aquarium a pike was 

 once placed in the same tank with a number of smaller fish. 

 The pike promptly swallowed his neighbors. A glass partition 

 was then put in, separating the pike from the small fish. The 

 pike would dart at them, however, and was often stunned by 

 striking the glass plate. But after a while he stopped darting 

 at the small fish ; and when the partition was removed, the pike 

 would always turn aside on approaching one of the little fish. 

 There was now nothing to prevent his eating them except 

 certain connections in his nervous system. 



This ability to form associations is of great practical impor- 

 tance to us not only in our control of lower animals but in our 



