50 HELEN DEAN KING 



of individuals in any two succeeding generations, although 

 a comparison of the behavior of rats in early and in late 

 generations indicates clearly the striking modifications that 

 occurred. 



A high nervous tension and extreme fear of man was shown 

 by all rats in early generations. They ran wildly about the 

 cage even at the approach of colony workers to whom they 

 were accustomed, and constantly gnawed the wire netting and 

 other parts of the cage in their efforts to escape from con- 

 finement. When the cage door was opened the rats often 

 jumped directly at the face of the person who thus seemed 

 to offer them a way to freedom. An escaped rat, when cap- 

 tured, showed marked evidence of fear, trembling and clicking 

 its teeth, and sometimes on being returned to the cage lay 

 inert for a few moments and then died. At this time con- 

 siderable difficulty was experienced in rearing the young. If 

 a nest was disturbed at or soon after the birth of a litter, the 

 mother usually destroyed her offspring. If the members of a 

 litter were removed from the nest, "even when they were sev- 

 eral days old, only the exceptional females would care for 

 them on their return. Many litters, therefore, had to be reared 

 by foster mothers from other strains. 



After the rats had become adjusted to new conditions of 

 life, their fear of man and nervous tension decreased greatly. 

 A rat that escaped from the cage displayed little resistance 

 on being captured, and its behavior was normal when it re- 

 joined the family group. As a rule, rats remained quiet when 

 the cage door was opened, even when they were being in- 

 spected by strangers. Many of them came to the front of the 

 cage at the approach of the feeding truck, and would take 

 food offered through the wire netting of the door. Females no 

 longer resented inspection of their offspring, and took excel- 

 lent care of them. If newborn young were removed from the 

 nest for examination, they would be cared for on their return. 

 Many females would rear litters from other strains if they 

 were barred from the nest until the young had acquired the 

 nest odor. However, alien young were always killed if the 



