LIMITS OF COMPARATIVE PSYCHOLOGY 725 



in which there is a dislocation in time between the action of an 

 outer agent and the reaction of the machine." This time- 

 adjustment is, in fact, merely a subordinate form of that general 

 ability to vary the response to a fixed environment, which consti- 

 tutes a non-mechanical, and thus an admittedly conscious activity. 



At last, the way has been cleared for the definite question, what 

 animals have been proved to react adaptively, that is, to learn? 

 To begin with the unicellular animals: Verworn 1 and Jennings 2 

 have shown that most of them make a fixed and utterly unvaried 

 response to a given environment. Many of them even react in but 

 one way to all surroundings, giving the same response to every stimu- 

 lus, however localized. The Paramecium, for example, as described 

 by Jennings, "responds to any stimulus by swimming forward. 

 . . . The direction of motion is the same whether the source of 

 stimulation is at the anterior end, the posterior end, the side. . . . 

 If the stimulus is at the posterior end [the animal] swims towards 

 it even though this results in ... destruction." Other organisms 

 of this class have more than one form of reaction, but respond 

 invariably by the same reaction to a given stimulus: the changed 

 response is, in other words, called out only by altered surround- 

 ings. In the Dileptus, for example, and the Loxodes forms of 

 Ciliata the reaction varies slightly with differently located mech- 

 anical stimuli: 3 a stimulus in front causes the Dileptus to swim 

 backward, whereas a posterior stimulus is followed by forward 

 motion. The essential feature of both types of reaction, of the 

 indiscriminate and the discriminate response alike, is the fact that, 

 in both cases, the movements are fixed and unvarying if the en- 

 vironment do not change. 



There is, however, at least one experimentally observed excep- 

 tion to the rule that unicellular animals react in this unvaried 

 fashion. This is the case of fixed infusoria, Steritor and Vorticella, 

 which not only vary their response to different stimuli, according 

 as these are harmful or beneficial; but which markedly alter their 

 response to a given stimulus, when it is prolonged. 4 



Observation and experiment have, however, mainly had to 

 do not with unicellular organisms but with animals of more com- 

 plex structure. Romanes and Loeb have studied the radiates. 

 Romanes attributes sense-consciousness both to jelly-fish and to 



1 Psycho-physiologische Protisten-Studien, Jena, 1889. Cf. pp. 137 seq., p. 141. 

 Cf. for similar studies of plants: Pfeffer, Untersuchungen aus dem botanischen 

 Institut, n, Tubingen, 1888. 



2 Studies on Reactions to Stimuli in Unicellular Organisms, n, The American 

 Naturalist, vol. 33, p. 386, 1899. 



3 H. S. Jennings, On the Movements and Motor Reflexes of the Flagellata and 

 Ciliata, Amer. Jour, of Phj/siol, vol. in, pp. 242 fteq., 1900. 



4 H. S. Jennings, Studies on Reactions to Stimuli in Unicellular Organisms, IX, 

 Amer. Jour, of Physiol., vni, pp. 23 seq., esp. 52 scq. 



