332 THE EVOLUTION OF ANIMAL BEHAVIOUR 
We regard reflex action and instinctive behaviour, broadly 
considered, as genetically prior to that which is intelligent. 
Their development in the individual and their evolution in 
the race are reached by the differentiation and integration of 
nerve-centres. In the abdominal region of the crayfish, for 
example, special centres are differentiated for the behaviour 
of each pair of swimmerets ; but these are so integrated that 
the whole series of like abdominal appendages swing rhythmi- 
cally with co-ordinated movements. Now, when a sensorium 
is developed, it docs not have to group by an act of con- 
scious selection and deliberate arrangement the multiplicity of 
scattered sensory data which it receives ; it does not have to 
organize from diverse and hitherto unrelated elements some 
sort of system in experience: it receives them as a physio- 
logical heritage already grouped, and to some extent organized. 
Stimulus and response are organically linked ; and within the 
response inherited co-ordinations, often exceedingly complex, 
afford a correlated group of sensory data. Just in so far as 
organic heredity has provided a working system of bodily 
parts, does consciousness receive systematic information of 
their orderly working. No doubt it is true that the develop- 
ment and evolution of the sensorium proceeds pari passu with 
the development and evolution of reflex actions compounded 
and co ordinated to give rise to instinctive behaviour. No 
doubt the progress of the one is in close touch and relation 
with the progress of the other; for such relation receives the 
emphatic sanction of utility. Suill it is none the less true 
that in individual development, as in racial evolution, the 
organic takes the lead. What is intelligently acquired is 
something added to that which has been engrained, through 
natural selection or otherwise, as a potentiality of the germinal 
substance. What we have first to note, then, is that organic 
evolution provides ready-grouped data to consciousness. 
The second point is, that the germs of abstraction and 
generalization, or rather processes which are the precursors of 
abstraction and generalization, arise, and cannot fail to arise, 
in the genesis of experience from the performance of inherited 
