150 THE PHILOSOPHY OF BIOLOGY 



extensive " bed " of young mussels forms on a part of 

 the sea bottom. In a short time the bed becomes 

 populated by a shoal of small plaice feeding greedily 

 on the little shellfish. In their peregrinations the 

 fishes must repeatedly pass out beyond the borders 

 of this feeding-ground. Usually, however, they will 

 return, for failing to find the food they like they swim 

 about in variable directions and so re-enter the shell- 

 fish bed. 



Suppose (this was really a fine experiment made 

 by Yerkes) a crab is confined in a box from which 

 two paths lead out but only one of which leads to the 

 water. The animal nins about at random, finds the 

 wrong path, retraces it, tries again and again, and 

 then finds the right path and gets back to the water. 

 If the experiment is repeated the animal finds the 

 right path again with rather less trouble, and after 

 many trials it ends by finding it at once on every 

 repetition of the experiment. 



^^ All this discussion of concrete cases leads up to 

 our consideration of the modes of acting in the higher 

 organisms. On the strictly mechanistic manner of 

 thinking the actions of the organism in general are 

 based on reactions of the tactic kind — inevitable re- 

 actions the nature of which is determined, and which 

 follow a stimulus with a certainty often fatal to the 

 organism displaying them. Accepting these tactic 

 reactions as, in general, truly descriptive of the be- 

 haviour of the organism, we can build up a theory 

 of instincts. In their simplest form instincts are 

 reflexes — tactic movements. In their more complex 

 forms they are concatenated reflexes, or tactes. A 

 complicated instinctive action is one consisting of 

 many individual actions, each of which is the stimulus 

 for the next one ; or, of course, it may also be complex 



