repetition is merely the negative case of the circular reaction. We
must not put too much of our own ideas into the author's mind; he
nowhere says explicitly that the animal or plant shows its sense and
does this because it likes the one thing and wants it repeated, or
dislikes the other and stops its repetition, as Butler would have
said. Baldwin is very strong in insisting that no full explanation
can be given of living processes, any more than of history, on purely
chemico-physical grounds.

The same view is put differently and independently by H. S. Jennings,
{0i} who started his investigations of living Protista, the simplest
of living beings, with the idea that only accurate and ample
observation was needed to enable us to explain all their activities
on a mechanical basis, and devised ingenious models of protoplastic
movements. He was led, like Driesch, to renounce such efforts as
illusory, and has come to the conviction that in the behaviour of
these lowly beings there is a purposive and a tentative character--a
method of "trial and error"--that can only be interpreted by the
invocation of psychology. He points out that after stimulation the
"state" of the organism may be altered, so that the response to the
same stimulus on repetition is other. Or, as he puts it, the first
stimulus has caused the organism to pass into a new "physiological
state." As the change of state from what we may call the "primary
indifferent state" is advantageous to the organism, we may regard
this as equivalent to the doctrine of the "circular reaction," and
also as containing the essence of Semon's doctrine of "engrams" or
imprints which we are about to consider. We cite one passage which
for audacity of thought (underlying, it is true, most guarded
expression) may well compare with many of the boldest flights in
"Life and Habit":-