70 HELEN DEAN KING AND HENEY H. DONALDSON 



4 per cent at the age period of twelve months and 38 per cent 

 at the age period of twenty months (table 10). In the later 

 generations the mortality at twenty months tended to be less 

 in females than in males (chart 11). 



14. Mortality decreased steadily as the generations ad- 

 vanced. In males it fell from 80 per cent in the first genera- 

 tion to 32 per cent in the tenth generation; in females the 

 drop was from 80 per cent to 19 per cent. The chief causes 

 of death were lung disease and malignant growths. 



15. Marked changes in behavior occurred during the course 

 of ten generations. The rats gradually lost much of their 

 savageness as well as their fear of man, and their nervous 

 tension decreased considerably. 



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