‘ 
162 INTELLIGENT BEHAVIOUR 
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 pari 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. 
When we compare, however, the several grades of intelli- 
gence which observation suggests, and when we watch the 
conscious development of the more intelligent animals, we seem 
to find evidence of the growth of a system of experience, at 
first in very close touch with inherited modes of procedure, 
but gradually acquiring more of independence and freedom. 
Increase of the range and complexity of behaviour brings with 
it, not only increase in the range and complexity of experience, 
but also—what is, perhaps, even more essential to effective 
progress—greater unity and closer connection into a well-knit 
whole. And with this greater unity and closer connection 
there goes what one may term a condensation of experience by 
an elimination of detail and the survival of essential features 
repeatedly emphasized. ‘This is analogous in the development 
of intelligence to the generalization and abstraction which play 
so important a part in the development of reason. It affords, 
in fact, the data which reflection utilizes in the purposive and 
* Supra, p 44, 
