I. LIFE PROCESSES 67 



bath destroys the nest odor of the rats and sterilization re- 

 moves all other animal odors from the cage and bedding. The 

 rats huddle together under the bedding for some time, dazed 

 by the ether and thoroughly cowed by the treatment they 

 have received. When they have become dry and have ven- 

 tured from hiding to inspect their new quarters, they are 

 usually friendly and do not fight subsequently. Even this 

 drastic treatment is not always effective, however, and after 

 this procedure it is not uncommon to find on the following day 

 that one or more of the individuals have been killed. 



By the end of ten generations in captivity, the behavior 

 of these gray rats had changed considerably in many re- 

 spects : they had lost much of their viciousness and their fear 

 of man ; females rarely killed the young returned to the nest 

 after being handled with bare hands; their nervous tension 

 was distinctly less than that in animals of the earlier genera- 

 tions, although they were obviously frightened when taken 

 from the cage to be weighed or when the cage was approached 

 by those to whom they were not accustomed; they made but 

 little effort to escape when the door of the cage was opened 

 or when the cage was cleaned. Rats of the later generations 

 had not, however, overcome their aversion to the presence of 

 strange individuals, and such intruders were killed as 

 promptly as in the earlier generations. 



Although gray rats of the later generations were somewhat 

 more nervous and more easily frightened than are the general 

 run of Albinos, they were far more vigorous and active, and 

 they tended to be larger animals, especially the females. 

 Sterility in these Grays was much less than that commonly 

 found in stock Albinos, and the two races were nearly equal 

 as regards fertility. The life span in the Grays compared 

 very favorably with that in the Albinos, and both r 

 showed about the same susceptibility to lung infection and 

 to other diseases. Because of their size and vigor, tlic>- 

 Grays would seem to be superior to Albinos for many labora- 

 tory purposes. When captivity has rendered them tame 

 enough to be handled easily, they should be valuable material 



