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 



