52 HELEN DEAN KING AND HENKY H. DONALDSON 



None of them had cast young up to this time, but they might 

 have done so later, since many females in the early genera- 

 tions did not breed until they were a year or more old. If 

 these females are omitted from the data in table 8, the sterility 

 for the females of the first generation falls to 31.48 per cent. 



The marked decrease in sterility in the offspring of wild 

 females is very significant, since it shows that the majority 

 of them quickly adapted themselves to conditions of life in 

 captivity, which had apparently rendered many of the wild 

 females impotent. Eats of this first generation showed far 

 less fear and less nervous tension than did their wild parents, 

 and all of the females reared their own offspring, which but 

 one of the wild females was able to do. 



Sterility decreased steadily as the generations advanced 

 (table 8), and at the fourth generation it had fallen to about 

 11 per cent, which is the percentage level of sterility in well- 

 cared-for stock Albinos (Slonaker and Card, '23). Sterility 

 reached its lowest point at the eighth generation, where it 

 was but 5.88 per cent. A trend to higher sterility in the stock 

 is not indicated by the increased sterility in the ninth genera- 

 tion. In this generation two of the females died before they 

 were six months old, two others had to be killed because they 

 had ovarian tumors, and three females in one litter developed 

 lung infection at an early age and did not breed although 

 they lived for several months. 



The striking decrease in sterility in succeeding generations 

 of these gray rats is shown by the graph in chart 9, con- 

 structed from data in table 8. 



From the downward trend of the graph in chart 9, it is 

 obvious that the conditions under which these gray rats lived 

 tended to eliminate sterility. In each generation the rats 

 became more adapted to their changed environment, and by 

 the third generation had lost much of their fear of man. They 

 came to the front of the cage when they heard the truck that 

 brought food, and were not frightened when the young were 

 taken out for examination or when the cage was cleaned. 

 The increased fertility in these rats as the generations 



