372 MARGARET FLOY WASHBURN 



r 

 with time. The first step is to determine how many times a 

 shadow of a definite duration, the light being otherwise kept 

 at a given constant intensity, must be made at a fixed rate 

 before the snail will cease to respond by drawing in its tenta- 

 cles. When this number has been found, the experiment is 

 repeated after a given interval, and if the interval is not too 

 long, the effects of the previous experience will be shown in 

 the fact that not so many repetitions of the shadow are now 

 required to produce cessation of the reaction. The longer the 

 interval, the smaller will be the amount of the saving in repe- 

 titions thus brought about. By determining the amount of 

 saving for different intervals, Pieron establishes the formula m 



k (log t) a 



(the mnemonic trace) = , where k, a, and /3 are con- 



t/3 

 stants and t is the interval of time elapsing since the first ex- 

 perience. Pieron thinks that the formula thus found is truer 

 even for human memory than that of Ebbinghaus, which is 



k 

 m = , and supports this statement by some experi- 



(log t) a 

 ments on the human subject. In his second paper (25), his 

 purpose is to find out what interval between successive shad- 

 owings will most quickly bring about cessation of reaction. He 

 finds that for the fresh-water snail the optimum interval is 

 from ten to twenty seconds, and that for Littorina it is as long 

 as one minute. In man, as has been indicated by the experi- 

 ments of Jost and others, it may be as long as twenty-four 

 hours. Whether the process of becoming adapted to repeated 

 stimulation really has anything in common with the processes 

 of establishing associations in the human cortex remains a 

 question. 



Daw T son's monograph on the biology of Physa (4) considers 

 in its first section the subject of habitat. The following condi- 

 tions are enumerated as those of an optimum environment: 

 "shallow water, minimum amount of shade, few or no enemies, 

 minimum amount of debris, protection from waves and cur- 

 rents, moderate amount of water weeds, and w 7 ell aerated water." 

 The second section is devoted to mucus and the spinning of 

 mucus threads. The common practice among water snails is, 



