170 BEGINNINGS OF INTELLIGENCE 
the states of consciousness which it endeavors to expel are 
the correlatives of beneficial actions, it must quickly disap- 
pear through persistence in the injurious and avoidance 
of the beneficial. In other words, those races of beings only 
can have survived in which, on the average, agreeable or 
desired feelings went along with activities conducive to the 
maintenance of life, while disagreeable and habitually- 
avoided feelings went along with activities directly or in- 
directly destructive of life; and there must ever have been, 
other things equal, the most useful and long-continued 
survivals among races in which these adjustments of feelings 
to actions were the best, tending ever to bring about perfect 
adjustment.” 
This explanation which has become widely accepted 
leaves a fundamental question unanswered. It does not 
explain why certain acts are stamped in and certain others 
stamped out. Of the mechanism of this process, which is 
the real problem involved in the pleasure-pain reaction, 
we are as ignorant as before. _The explanation means that 
animals which took pleasure in following acts that brought 
them benefit were preserved and those that did not behave in 
this manner were eliminated. But why does an animal tend 
to repeat an act that brings it pleasure and avoid one that 
produces pain? Itseemsso natural for creatures to behave 
in this way that the existence of any problem here is usually 
unsuspected, but this is the problem that confronts us when 
we endeavor to obtain a clear understanding of the way 
in which intelligence develops out of instinct. 
In the pleasure-pain response we have two problems of a 
quite different nature: (1) the problem of how behavior is 
modified by its results, and (2) the problem of why pleasure 
is associated with certain physiological activities, such as 
securing movements, and pain with others, such as avoiding 
