sary to cause the leaves to close, and the two must not be more than three minutes apart; the effects of the first stimulus are in some way stored or registered in the leaf for this brief time. This kind of phenomenon is widespread among living things and is known as "summation of stimuli." In all such cases the effects of a former stimulus are in some way stored up for a longer or shorter time in the protoplasm. It is possible that this is the result of the formation of some chemical substance which remains in the protoplasm for a certain time, during which time the effects
Fig. 20. Dionæa muscupula (Venus' Flytrap). Three leaves showing marginal teeth and sensitive hairs (SH). The leaf at the left is fully expanded, the one at the right is closed.
of the stimulus are said to persist, or it may be due to some physical change in the protoplasm analogous to the "set" in metals which have been subjected to mechanical strain.
Probably of a similar character is the persistence of the effects of repeated stimuli and responses on any organ of a higher animal. A muscle which has contracted many times in a definite way ultimately becomes "trained" so that it responds more rapidly and more accurately than an untrained muscle; and the nervous mechanism through which the stimulus is transmitted also becomes trained in the same way. Indeed such training is probably chiefly a training of the nervous mechanism. The skill of the pianist, of the tennis player, of the person who has learned the difficult art of standing or walking, or the still more difficult art of talking, is probably due to the persistence in muscles and nerves of the effects of many previous activities. All such phenomena were called by Hering, "organic memory," to indicate that this persistence of the effects of previous activities in muscles and other organs is
