Simple creatures act with a certain degree of spontaneity on their
environment, and they likewise react effectively to surrounding stimuli.
Animals come to have definite "answers back," sometimes several,
sometimes only one, as in the case of the Slipper Animalcule, which
reverses its cilia when it comes within the sphere of some disturbing
influence, retreats, and, turning upon itself tentatively, sets off
again in the same general direction as before, but at an angle to the
previous line. If it misses the disturbing influence, well and good; if
it strikes it again, the tactics are repeated until a satisfactory way
out is discovered or the stimulation proves fatal.
It may be said that the Slipper Animalcule has but one answer to every
question, but there are many Protozoa which have several enregistered
reactions. When there are alternative reactions which are tried one
after another, the animal is pursuing what is called the trial-and-error
method, and a higher note is struck.
There is an endeavour after satisfaction, and a trial of answers. When
the creature profits by experience to the extent of giving the right
answer first, there is the beginning of learning.
[Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
LIKE AN EARTHWORM