The Instinctive Reactions of Young Chicks 167 
definite particular acts invariably done in the presence of 
certain equally definite situations. Such an act as the 
spider’s web-spinning might be a stock example. Of 
course, there are many such instinctive reactions in which a 
well-defined act follows a well-defined stimulus with the 
regularity and precision with which the needle approaches 
the magnet. But our experiments show that there are acts 
just as truly-instinctive, depending in just the same way on 
inherited brain-structure, but characterized by being vague, 
irregular, and to some extent dissimilar, reactions to vague, 
complex situations. 
The same stimulus doesn’t always produce just the same 
effect, doesn’t produce precisely the same effect in all in- 
dividuals. The chick’s brain is evidently prepared in a 
general way to react more or less appropriately to certain 
stimuli, and these reactions are among the most important 
of its instincts or inherited functions. But yet one cannot 
take these and find them always and everywhere. This 
helps us further to realize the danger of supposing that in 
observation of animals you can depend on a rigid uniform- 
ity. One would never suppose because one boy twirled 
his thumb when asked a question that all boys of that age 
did. But naturalists have been ready to believe that 
because one young animal made a certain response to a cer- 
tain stimulus, the thing was an instinct common to all in pre- 
cisely that same form. But a loud sound may make one 
chick run, another crouch, another give the danger call, and 
another do nothing whatever. 
In closing this article I may speak of one instinct which 
shows itself clearly from at least as early as the sixth day, 
which is preparatory to the duties of adult life and of no 
other use whatsoever. It is interesting in connection with 
the general matter of animal play. The phenomenon is as 
