were as follov.s; - 



Age at which work began number of rats in the grou>v 



25 days 8 7 



65 " 27 



200 " 28 



300 " 28 



500 " 10 



Throughout the exi.eriment two conditions have been rigid- 

 ly conii lied vath. 1st : Every animal was started on the problem 

 ui on the exact day at which the proper age was reached. This 

 procedure was follov.ed even when it necessitated starting 

 eighteen rats on the same day, in order that exj erinental con- 

 ditions night be kej: t strictly comparable. 



2ad: Kvery animal was run twice every day fron its first 

 trial until tiie last, even though at one stage of the work this 

 required having as many as fifty -eight rats under observation 

 at one time. Such strict continuity of trials precluded the 

 introdiiction of any factors aside from those involved in the 

 learning process proper. Removal from experimental conditions 

 for even one day would not only cause a change in the physiolog- 

 ical tonus of the organism, but would also bring in the matter 

 of retention. 3o far as was possible the rats were run at the 

 same hour each day, but where large numbers were being used it 

 was impossible to adrere strictly to this rule, although rats 

 accustomed to run at night did not do so well if used in the 

 morning and vice versa; which was probably attributable to the 

 acquiring of a certain food rythm that might not be broken with 

 impunity. Thus it was found that while a difference of an hour 



