HABIT FORMATION IN THE ALBINO RAT 13 



months, 22 and Donaldson gives it as three years, from three 

 hundred to four hundred days is the maximum longevity for 

 most of the rats used in this laboratory, and up to this time 

 only twelve have lived to work at the five hundred day age, 

 one of these dying apparently of old age before the problem was 

 learned. 23 



The groups used, with the number of rats in each group were 

 as follows: 



Age at which work began Number of rats in the group 



Throughout the experiment two conditions have been rigidly 

 complied with. 1st: Every animal was started on the problem 

 upon the exact day at which the proper age was reached. This 

 procedure was followed even when it necessitated starting 

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

 ditions might be kept strictly comparable. 



2nd: Every animal was run twice every day from its first 

 trial until the 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 

 introduction 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. So 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 adhere strictly to this rule, although rats 

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

 mornings and vice versa; this was probably attributable to the 

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

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

 in the working time, and hence the feeding time, caused no 



22 Op. cit., Joitrn. Animal Behav., pp. 37-38, tables. 



23 Dr. Watson has informed the writer that his experience was quite similar, 

 very few of his rats living to be more than 500 to GCiO days old. 



