Associated Stimuli. 
201 
memory in the sense of conscious reminiscence, but simply the re¬ 
call of a previous condition of the organism. It must be noted that 
on the second occasion, when the dog runs at the sight of the boy 
stooping, there is repetition of the direct stimulus and also an 
awakening of the engram I., so that there is ecphory of I., as well of 
of its associated engram P. We may imagine at a third or fourth 
repetition of the stone-throwing experience that the image of a boy 
—a good boy who has no intention of picking up a stone, will be 
sufficient to act as a stimulus. That is to say a small part of the 
original stimulus, will revive the whole of the now deeply engraved 
engrams I. and P., and as before the dog will run. 
These cases are of interest because they seem to indicate a 
way in which organisms can become more sensitive to any stimulus 
by its repetition. It is of importance to an organism which has to 
exist in a complex and hostile environment that it should be highly 
sensitive to external change, and especially that it should perceive 
a coming danger before it has reached a hurtful intensity. No 
doubt this heightened sensitiveness is selected like other adaptations. 
But we see here that without the existence of variation in 
sensitiveness there is in the mnemic capacity a possibility of 
automatic increase of sensitiveness. 
An instance of associated engrams occurs in the reactions of 
the gemmai of Marchantia, investigated by Pfeffer. 1 The gemma has 
dotted over its surface certain cells, some of which finally develop 
into rhizoids. The question which shall so develop is settled by 
at least two factors—sensitiveness to gravity and to contact 2 . 
In the ordinary course of events the gemma falls on the 
ground and the root-hairs grow on the physically lower surface. 
This surface is also that on which contact occurs. In Pfeifer's 
experiments the gemmae were fixed to the roof of a damp chamber, 
roots appeared on both upper and lower surfaces. 
We may suppose that the contact-irritability and the gravita¬ 
tional sensitiveness may have been independently evolved. But the 
point I wish to make is that if one of these qualities were developed 
the other would follow by association of engrams. If for instance 
the root-hairs were originally developed exclusively in response to 
gravity, i.e., on the physically lower side, the growth of the hairs 
would be accompanied in the majority of cases by contact on the 
1 Sach’s Arbeitcn, I., 77. 
2 Benecke, B.Z., 1906, doubts whether the stimulus is actually 
contact. It seems probable that the reaction depends on the 
presence of water containing salts in solution. 
