vii ASSIMILATION AND READJUSTMENT 141 



where it had been previously kept crawled at once to the 

 surface, while snails freshly taken from the pond wandered 

 aimlessly. 1 Professor Yerkes has definitely shown that 

 worms can learn a path, forming a habit as the result 

 of from twenty to one hundred experiences, and, what is 

 perhaps most remarkable, the habit is independent of the 

 so-called brain, that is the ganglia situated in the anterior 

 segments, since it persisted when these were removed. 2 

 Finally, with regard to protozoa, we have definite in- 

 stances of selective modification, e.g.^ the behaviour 

 of stentor quoted above (p. 114) has been sufficiently 

 described. For the direct reversal of a response, due 

 to the unsuitable character of the object, the evidence is 

 less clear. The most definite case recorded is an experi- 

 ment by Messrs. Hodge and Aiken, who gave some 

 Vorticellae, a " pure culture of yeast plants," in place 

 of their normal food. "This attempt," they say, "re- 

 sulted in an interesting demonstration of the educability 

 of vorticellae." They took greedily to the food at first, 

 then ejected it " with volcanic energy," and afterwards 

 " for several hours at least how long the memory lasted 

 was not determined the individual could not be induced 

 to repeat the experiment." 3 One would be glad to know 

 whether during the same time they took any other food. 

 It would seem possible that they were left in a state of 

 collapse. 



Metalnikow adduces a series of experiments on 

 Paramecia, showing that at first these infusoria swallow 

 injurious substances in large quantities, but, at the end 

 of one or two days at most, they cease to do so while 

 still swallowing other substances. " It seems," he said, 

 " that the less nutritious the substance the more quickly 

 the distinction is learnt." But it is not clear that it was 

 the same Paramecia. Metalnikow's method of testing the 

 condition of the Paramecia was to take a drop from 

 the culture on successive occasions and kill the specimens, 



1 The Biology of Physa, summarised in J. A. B., 1912, p. 373. 



2 R. M. Yerkes, The Intelligence of Earthworms, J. A. B., 1912, 

 especially pp. 351-2. 



3 Quoted by Mr. G. P. Watkins, " Psychical Life in Protozoa," American 

 Journal of Psychology, Jan. 1900, pp. 179, 180. 



