substance as carmine ink be introduced into the water so as to reach Stentor's disk there are several reactions. At first the normal movements of the cilia which cause a current of water to flow toward the animal's mouth are not changed. The particles of carmine ink enter the mouth and thence penetrate the internal protoplasm. If the cloud becomes dense, however, the animal presently bends aside. If this reaction is not effective in getting rid of the particles it is repeated. If failure still results the ciliary movements are suddenly reversed to produce a current of water away from the mouth. This reversal is brief but, if no improvement is effected, it may be repeated many times in rapid succession. Next, contraction within the sheath may occur. By this contraction the animal escapes stimulation entirely, but it also obtains no food. Usually the animal extends itself again in less than a minute. If particles of carmine are again met Stentor no longer reacts in the milder ways employed at first, but contraction occurs at once. This may be repeated many times, each period of retirement lasting longer than the preceding one. Ultimately the animal contracts repeatedly and violently while still encased in its tube. It thus finally detaches its foot from its moorings, leaves its tube, swims away, attaches itself elsewhere and forms a new sheath in a new and more favorable environment.
This behavior differs from that of Paramecium in a radical way. Paramecium, except when fatigue or other cause reduces surplus energy, always reacts in the same way to the same stimulus. Stentor reacts in different ways. As Jennings puts it "the change in reaction must be due to a change in the organism" itself. In any event the present readiness of this organism to react in one of two or more possible ways depends on its past history. The animal profits by experience. The change in reaction is regulatory, not haphazard. Something akin to habit has appeared. This clearly marks the saving of much energy that otherwise would be spent in useless attempts to avoid injurious conditions. There is less waste and more surplus, more chance of survival and a greater length of time during which the animal possesses a surplus of energy over the minimum necessary for survival.
To trace the gradual development of creatures more complex in structure, and in consequence more complex in behavior, might be interesting, but if the recent work of observers of animal behavior is to be trusted no new principles are involved until the primates themselves are approached.
Throughout the period marked by this interval, however, countless changes in the structure, in the production of new reflexes, in the modification of instinctive behavior by the formation of habits, perfected the adjustment of individuals to the physical environment and their accommodation to their fellow creatures. Definite complex com-