akin to that persistence of the effects of previous experiences in the nervous mechanism which we commonly call memory. It seems probable that this ability of protoplasm in general to preserve for a time the effects of former stimuli is fundamentally of the same nature as the much greater power of nerve cells to preserve such effects for much longer periods and in complex associations, a faculty which is known as associative memory. The embryos, and indeed even the germ cells of higher animals, may safely be assumed to be endowed with protoplasmic and organic memory, out of which, in all probability, develop associative and conscious memory in the mature organism.
4. Intellect, Reason.—Even the intellect and reason which so strongly characterize man have had a development from relatively simple beginnings. All children come gradually to an age of intelligence and reason. In its simpler forms at least reason may be defined as the power of predicting future events and of reaching conclusions regarding unexperienced phenomena under the influence of past experience. In the absence of individual experience young children have none of this power, but it comes gradually as a result of remembering past experiences and of fitting such experiences into new conditions. Young infants and many lower animals lack the power of reason, though their behavior is frequently of such a sort as to suggest that they are reasoning. Even the lowest animals avoid injurious substances and conditions and find beneficial ones; more complex animals learn to move objects, solve problems, and find their way through labyrinths in the shortest and most economical way; but this apparently intelligent and purposive behavior has been shown to be due to the general elimination of all sorts of useless activities, and to the persistence of the useful ones.
The ciliated infusorian, Paramecium, moves by the beating of cilia which are arranged in such a way that they drive the animal forward in a spiral course. However, when it is strongly irritated, the normal forward movement is reversed; the cilia beat forward instead of backward and the animal is driven backward for some distance (Figs. 21, 1, 2, 3); it then stands nearly still merely rolling over and swerving toward the aboral side and finally it goes ahead again, usually on a new course (Fig. 21, 3, 4, 5, 6). These movements seem to be conditioned rather rigidly by the organization of the animal: they are more or less fixed and mechanical in character though to a certain extent they may be modified by experience or physiological states. Paramecium behaves as it does in virtue of its constitution, just as an egg develops in a particular way because of its particular organization.
But although limited in its behavior to these relatively simple motor reactions, Paramecium does many things which seem to show intelligence and purpose. It avoids many injurious substances, such as strong salts or acids and it collects in non-injurious or beneficial substances, such as weak acids, masses of bacteria upon which it feeds, etc. It avoids ex-
