110 INSTINCT 



energies in instinctive acts. When the spider spins its web, 

 when the wasp digs a hole and stores it with a certain kind 

 of prey for its young, and when the bird builds its nest, and 

 the beaver its dam there is of course response to certain 

 features of the environment; there is also an innate propen- 

 sity for the organic machinery to work in certain ways, 

 much as a piece of clock work runs in a particular fashion 

 after it has been wound up and set going. 



Activity which is internally initiated is not fundament- 

 ally different from activity which we commonly call reflex; 

 the stimuli by which it is evoked are internal instead of 

 external; they result in many cases from the rhythms of 

 organic functions, chance discharges of nervous energy 

 due to various physiological changes, and various other 

 factors. Such activities are to a high degree characteristic 

 of particular species and are doubtless as rigidly determined 

 by organization as are the direct responses to external 

 stimuli. 



The nature of the instinctive act that may be performed 

 in a given situation is notoriously dependent upon the 

 internal condition of the animal. The same stimulus may 

 evoke in different states quite contrary impulses. The 

 sight and smell of food may arouse an animal to vigorous 

 efforts to secure it, or produce feelings of aversion and 

 movements of avoidance, according to the creature's state 

 of hunger or satiety. The sexual behavior of animals is 

 dependent to a very marked degree upon internal conditions 

 which are correlated with the production and maturation 

 of the sex cells. Salmon begin their up-stream migrations, 

 the male frog develops his tendency to clasp the female; 

 birds herald the advent of the breeding season with court- 

 ship and song, and the males of many mammals show at 

 this time an unusual degree of belligerency. The change 



