EVOLUTION OF INTELLIGENT BEHAVIOUR 161 
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 and experiment seem to afford little indication 
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 
the view already indicated,* that the nerve-centres which are 
concerned in the conscious control begotten of experience are 
independent of those primarily concerned in normal reflex 
action, we may perhaps believe that the simplest nervous 
system, worthy of the name, contains both these elements, and 
that in the course of the evolution of nervous systems in 
higher and higher grades, there go on part passu the further 
differentiation of these elements, and the progressive integra- 
tion of reflex and control centres into a closely connected and 
effective whole. Not that any expression of the facts, if such 
they be, in terms of an evolution formula, adds anything to 
our knowledge of the organic modus operandi. We know 
but little of the intimate nerve physiology of even the highest 
invertebrates. We see ample evidence of the control of 
behaviour in the light of individual experience. Of any 
detailed knowledge concerning the manner in which this 
control is effected we do not seem to possess more than the 
rude initial phases. 
* Supra, p. 44. 
