lOO THE SOCIAL LIFE OF ANIMALS 



peated in our laboratory where significant steps have 

 been taken towards its further analysis. 



Vetulani, the original experimenter, (117) used 

 closely inbred mice for his experimental animals. He 

 measured the growth of males and females separately 

 from the sixth and on through the twenty-second 

 weeks of their lives. After rearrangement he followed 

 them for ten weeks longer as a sort of control. Fresh 

 food was supplied in abundance each day, and proper 

 experimental conditions seem to have been main- 

 tained. 



Growth during the first sixteen weeks of the ex- 

 periment is shown in the accompanying graphs (Fig- 

 ure 16). All started off at approximately the same 

 rate. After the fifth week of the experiment, however, 

 it is clear that the isolated mice were growing most 

 slowly, and they continued to do so as long as the 

 experiment ran. The most rapid rate of growth was 

 observed in those mice which were placed two to 

 four per cage; those five to six per cage grew next 

 best, and only slightly below these came those living 

 nine to twelve per cage. 



Under the conditions of this experiment the iso- 

 lated young mice were most handicapped, those most 

 crowded were next, while those that were somewhat 

 but not too crowded grew most rapidly. When the 

 mice were rearranged for a continuing period of ten 



