242 BIOLOGY AND HUMAN LIFE 



intelligence which enables the animal a'lways to do what is best 

 for it under given circumstances. That kind of "instinct" is 

 largely a myth. A frog would starve to death with hundreds of 

 dead worms and insects all about him, because his eating move- 

 ments can be started only by the sight of a moving object. Or the 

 frog will swallow a bit of cloth that is dangled in front of him, and 

 that has no food value whatever. Again, a female fly will lay her 

 eggs on a piece of paper that has been soaked in meat juice, al- 

 though this is extremely wasteful of eggs. Of course, in a state 

 of nature the only things that smell like meat or manure are 

 meat and manure ; and if the eggs are deposited in such mate- 

 rials, the young will be supplied with food. These instincts are, 

 on the whole, beneficial, or at least not fatal, to the species. 



We have seen that it is impossible for all animals to have per- 

 fect reflexes (see section 183). The same principle is true with 

 respect to instinctive acts of all kinds. The relations of living 

 things to one another is such that they cannot all get the food 

 they need and at the same time escape being eaten by others. 



199. Instincts can be modified. Eating is so fundamental to 

 keeping alive that we should expect instinctive activities related 

 to this process to be very well fixed in the constitution of an or- 

 ganism. We cannot teach a frog to eat food that is at rest, or to 

 ignore useless danghng bait ; yet eating instincts can be modified 

 in various ways. The dog refrains from eating after he has had 

 a good meal ; that is, when he is no longer hungry, the chemical 

 condition of his blood and of the other juices is different from 

 what it was, and the "hungry" nerves and muscles behave in 

 one way in the presence of food, whereas the muscles and nerves 

 of an organism that is not hungry behave in a different way. 

 If a goose has its food stuffed down the throat for several days, 

 the animal is no longer stimulated by the sight of grain etc. to 

 open the beak and take up food. 



In an aquarium a pike was placed with a number of smaller 

 fish. The pike swallowed his neighbors. A glass partition was 

 then put in, separating the pike from the smaller animals. The 

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



