EVOLUTION OF INTELLIGENT BEHAVIOUR 161 
avoiding its enemies. If the animal 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 (@) under 
compulsion, as When it is her last resort in self-defence ; 
(6) under a milder provocation, as one of three courses of 
behaviour, 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 ; (¢) or finally, wnder no special external stimulus, but 
rather from internal motive, the normal demand for rest and 
shady seclusion, presumably very strong in Clepsine after 
gorging itself with the blood of its turtle host.” 
Professor Whitman rightly regards the act of rolling into 
a ball ag instinctive, and due to natural selection. But he does 
not undertake to discuss the question as to how much intelli- 
gence, if any, Clepsine may have. Nor, indeed, is it an easy 
matter to determine. ‘The differential reaction according as 
the animal has eggs or not suggests intelligence ; but it may 
be instinct varying according to the conditions of stimulation 
external and internal. The different behaviour which may be 
seen in different cases when a stone is turned to the light 
again suggests intelligence, but again may be determined 
directly by the conditions of stimulation. Prompted by Dr. 
Whitman’s observations, I endeavoured to determine whether a 
leech would grow accustomed to frequent gentle stimulation 
with a camel’s-hair brush, and cease to react under circum- 
stances which were followed by no ill effects. But though I 
incline to think that this is the case, the observations were not 
such as to be satisfying and convincing. If intelligence 
be present we seem to find it in an early and rudimentary 
state. 
Observation, we must confess, seems to afford little indi- 
cation of the conditions under which intelligence first makes 
its appearance in the animal kingdom. And if we turn to 
general considerations, which at the best afford uncertain 
guidance, little light is thrown on the subject. If we accept 
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