han in the case of living organisms, and it is far less difficult
to invent satisfactory hypotheses as to the microscopic changes of
structure which mediate between the past occurrence and the present
changed response. In the case of living organisms, practically
everything that is distinctive both of their physical and of their
mental behaviour is bound up with this persistent influence of the past.
Further, speaking broadly, the change in response is usually of a kind
that is biologically advantageous to the organism.

Following a suggestion derived from Semon ("Die Mneme," Leipzig, 1904;
2nd edition, 1908, English translation, Allen & Unwin, 1921; "Die
mnemischen Empfindungen," Leipzig, 1909), we will give the name of
"mnemic phenomena" to those responses of an organism which, so far as
hitherto observed facts are concerned, can only be brought under causal
laws by including past occurrences in the history of the organism as
part of the causes of the present response. I do not mean merely--what
would always be the case--that past occurrences are part of a CHAIN of
causes leading to the present event. I mean that, in attempting to state
the PROXIMATE cause of the present event, some past event or events
must be included, unless we take refuge in hypothetical modifications of
brain structure. For example: you smell peat-smoke, and you recall some
occasion when you smelt it before. The cause of your recollection, so
far as hitherto observable phenomena are concerned, consists both
of the peat smoke (present stimulus) and of the former occasion (past
experience). The same stimulus will not produce the same recollection
in another man who did not share your former experience, although the
former experience left no OBSERVABLE traces in the structure of the
brain. According to the maxim "same cause, same effect," we cannot
therefore regard the peat-smoke alone as the cause of your recollection,
since it does not have the same effect in other cases. The cause of
your recollection must be both the peat-smoke and the past occurrence.
Accordingly your recollection is an instance of what we are calling
"mnemic phenomena."

Before going further, it will be well to give illustrations of different
classes of mnemic phenomena.

(a) ACQUIRED HABITS.--In Lecture II we saw how animals can learn by
experience how to get out of cages or mazes, or perform other actions
which are useful to them but not provided for by their instincts alo