44 



the problenn, preliminary feeding being thua exteniecL bectuse it 

 was found that rats so old contracted digestive troubles unless 

 the decrease in food supply was mala more gradual thhn for the 

 younger animals. They were allowed t- eat for from six to eight 

 minutes instead of five, at the close of each day's work, since 

 they ate much slower than the younger rats, and could not obtain 

 sufficient nourishment in the shorter time. These rats differ- 

 ed markedly in behavior from those in any of the preceding groups 

 in that they were lethargic, inactive, and often went to sleep 

 in the naze instead of working at their problem. A few of the 

 animals of this group were from The 'Vis tar Institute, and were 

 somewhat timid and difficult to handle, but even among animals 

 bred in this laboratory the same disinclination to v/ork was noted 

 although with our own rats it id not last so long. V/hen the 

 rats finally begun to work, they went about it differently from 

 those of other groups. They were very deliberate, follov.ed the 

 culs de sac out to the bitter ead whereas the other rats often 

 turned back toward the true path before reaching the alley stop, 

 furthermore, they did not hesitate to enter the unexplored run- 

 ways as dii most of the other rats, in this last respect resemb- 

 ling the twenty-five day rats. 



The trials varied from fourteen to eighty-four, absolute 

 time from five and eight tenths seconds to thirty-five and two 

 tenths seconds, total time from one hundred nine minutes to two 

 thousand two hundred seventy-foir minutes, and total distance 

 from one hundred seventeen and three tenths meters to six hun- 

 dred nine and six tenths meters. 



