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 
