DEVELOPMENT OF BEHAVIOR 315 



following discussion of development is based on the assumption that the 

 law is one of general validity. It must be kept in mind that this is 

 partly an assumption, but the probability that this will be found true 

 is such that the relation of development to the law is worth setting forth. 

 There is no other need greater in the study of animal behavior than 

 that of a thorough investigation of the validity of this law in the lower 

 organisms. 



The question in which we are here interested is then the following : 

 How can behavior develop ? That is, how can it change so as to become 

 more effective more regulatory? 



(1) The behavior of any organism may become more effective 

 through an increased tendency for the first weak effects of injurious or 

 beneficial agents to cause the appropriate reaction; in other words, 

 through increased delicacy of perception and discrimination on the part 

 of the organism. Such a change would be brought about through the 

 law of the readier resolution of physiological states after repetition. 

 When the organism is subjected to a slight stimulus, this changes its 

 physiological state, though perhaps not sufficiently to cause a reaction. 

 Such a slight stimulus would be produced by a very weak solution of a 

 chemical, or by a slight increase in temperature. Now, suppose that 

 this weak stimulus, causing no reaction, is regularly followed by a 

 stronger one, as would be the case if the weak chemical or slight warmth 

 were the outer boundary of a strong chemical solution, or of a region 

 of high temperature toward which the organism is moving. This 

 stronger stimulus would produce an intense physiological state, corre- 

 sponding to a marked negative reaction. That is, the first (weak) 

 physiological state is regularly resolved by the action of the stimulating 

 agent into the second (intense) one, inducing reaction. In time the 

 first state would come to resolve itself into the second one even before 

 the intense stimulus had come into action. As a result, the organism 

 would react now to the weak stimulus, as it had before reacted only to 

 the strong one. It would thus be prevented from entering the region 

 of the chemical or the heat, even before any injury had arisen. 



(2) In the same way the organism may come to react positively or 

 negatively to a stimulus that is in itself not beneficial nor injurious, but 

 which serves as a sign of a beneficial or injurious agent, because it regu- 

 larly precedes such an agent. Suppose that a slight decrease in illumina- 

 tion (a shadow), which is of itself indifferent, regularly precedes the ap- 

 proach of an enemy, as happens in the sea urchin. The slight decrease 

 in light induces a certain physiological state, which is so little marked 

 that in itself it produces no reaction. But through the immediately 

 following attack of the enemy, this indifferent physiological state is 



