I. LIFE PROCESSES 



53 



advanced can be ascribed, I believe, in part to the lessen inu 

 of nervous tension, and in part to the fact that the animals 

 were fed on a well-balanced ration containing the elements 

 essential for reproduction. 



70 



60 



50 



30 



20 



10 



Percent . Sterility 



W 1 2 3 4 5 



Chart 9 Showing the percentage of sterility in different generations of 

 captive Grays. 



Most, if not all, of the sterility found in females of the 

 later generations can be attributed to diseases which affected 

 the reproductive organs or so lowered vitality that the indi- 

 viduals could not breed. Autopsies were not made on rats 

 found dead in the cage, but when it was necessary to kill any 

 individuals because of obvious illness, the fact was noted. 

 Records show that a total of sixteen females belonging in the 

 first nine generations were killed because they had tumors, 



