
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 
