148 S. J. HOLMES 



no longer attached to objects that are found to be harmless, while 

 it may be intensified in relation to other objects that are found to 

 be sources of injury. Where there is hesitation between the exer- 

 cise of two instincts such as the tendency to pursue an animal as 

 prey, and the instinctive fear which that animal may awaken, 

 experience may quickly point out which proclivity is the more 

 advantageous to follow. The pleasure-pain reaction enables an 

 animal to select, so to speak, out of its stock of instinctive endow- 

 ments those responses which are best adapted to the particular 

 situations that confront it. It is a means of adapting instincts 

 to new or inconstant conditions and thus of effecting a closer 

 adaptation to the environment than that which would be possible 

 by following purely congenital modes of response. The develop- 

 ment of the pleasure-pain reaction marks one of the most impor- 

 tant steps in the evolution of behavior, for the entire superstruc- 

 ture of intelligence in all its stages is based upon it, and it is not 

 surprising that many writers regard it as an index of the begin- 

 ning of consciousness, a point where a new entity is somehow mys- 

 teriously injected into the universe. 



It is a general rule that what is pleasant is beneficial and what 

 is painful is injurious; and, therefore, by following its desires and 

 aversions an animal is guided in a tolerably safe course. Eating 

 when hungry, drinking when thirsty, seeking warmth when cold, 

 exercise when in a state of vigor, and rest when fatigued, all bring 

 a state of satisfaction or pleasure. On the other hand, eating 

 and drinking after a certain stage of repletion has been reached, or 

 attaining too great a degree of warmth may be positively painful, 

 the pain being correlated with carrying on these activities until 

 they become injurious to the organism. 



But it is well known that this correlation of the pleasant with 

 the beneficial is not an absolute one. With complex creatures 

 like ourselves with a multitude of different propensities and inter- 

 ests it is not infrequent that the pursuit of what is agreeable leads 

 to all sorts of unfortunate consequences even of a purely physio- 

 logical nature. In the lower animals where pleasure is a safer 

 guide than among ourselves, what is pleasant is not always what 

 is organically good. Poisonous articles may be eaten with appar- 



