INSTINCT 111 



in instinctive behavior during the breeding season may be 

 due to the production of internal secretions which influence 

 the irritability of certain parts of the nervous system, but, 

 however caused, it is, like the varying responses to food, 

 water, etc., pretty closely subservient to the needs of the 

 species. 



This dependence of behavior upon internal conditions 

 naturally increases the range of its possible adaptations. 

 Animals are often endowed with adaptive responses cor- 

 responding to this, that, or the other internal state. Pre- 

 vious exercise and many other factors change these internal 

 states, so that what an animal may do in a given situation 

 is not to be inferred from the external conditions alone. If 

 one response does not suit the animal tries another, and so 

 on. The condition of the animal is changed after one or 

 more reactions and this change produces a different reaction 

 to the stimulus. 



According to Whitman, the leech Clepsine when it is 

 stimulated may roll into a ball, hug the bottom, or crawl 

 away. "If the leech has eggs it will not roll up, but if it 

 has no eggs, or if it has young, it may adopt either mode of 

 escape, while if it has eggs it has no choice but to remain 

 quiet over them. The act of rolling up into a passive ball 

 may be performed (a) under compulsion, as when it is her 

 last resort in self defense; (b) under a milder provocation, 

 as one of three courses of behavior, as when the resting place 

 is turned up to light, and the choice is offered between 

 remaining quiet in place, creeping away at leisure, or rolling 

 into a ball and dropping to the bottom; (c) or finally, under 

 no special external stimulus, but rather from internal motive, 

 the normal demand for rest and seclusion, presumably very 

 strong in Clepsine after gorging itself with the blood of 

 its turtle host." "The differential reaction," says Lloyd 



