When we take a survey of animal behaviour we see a long inclined plane.
The outer world provokes simple creatures to answer back; simple
creatures act experimentally on their surroundings. From the beginning
this twofold process has been going on, receiving stimuli from the
environment and acting upon the environment, and according to the
efficiency of the reactions and actions living creatures have been
sifted for millions of years. One main line of advance has been opening
new gateways of knowledge--the senses, which are far more than five in
number. The other main line of advance has been in most general terms,
experimenting or testing, probing and proving, trying one key after
another till a door is unlocked. There is progress in multiplying the
gateways of knowledge and making them more discriminating, and there is
progress in making the modes of experimenting more wide-awake, more
controlled, and more resolute. But behind both of these is the
characteristically vital power of enregistering within the organism the
lessons of the past. In the life of the individual these enregistrations
are illustrated by memories and habituations and habits; in the life of
the race they are illustrated by reflex actions and instinctive
capacities.