LIFE PROCESSES IN CAPTIVE GRAY RATS 63 



from the competitive factor of natural selection, are used as 

 parents, would provide a far greater chance for mutations to 

 occur than would natural conditions. 



In this hypothesis may lie the explanation for the appearance 

 of various mutations in captive gray rats after they had been 

 under domestication for several years. 



In early generations the breeding of gray rats was restricted 

 mainly to the middle and most active period of reproductive 

 life, therefore germ cells in young and old individuals had 

 little chance to function. Females of the first nine generations 

 did not cast their first litters until they were, on the average, 

 over 7 months of age, and reproductive life averaged less than 

 8 months. In the period when mutations appeared breeding 

 began at an earlier age and was continued much longer than 

 in earlier generations (table 7). Three of the mutant types 

 obtained, black, albinism, and ruby-eyed dilute, first appeared 

 in litters cast by females that were from 14 to 16 months old, 

 and so near the end of reproductive life. The first curly 

 mutant was born when the mother was about 6 months of age. 

 These findings, although forming but a very small series, are 

 in accord with Little 's hypothesis. Other investigations along 

 this line may further confirm it. It may be true that ''muta- 

 tions are accidents and accidents will happen" (Sturtevant, 

 '37), but to ascertain the basic cause of such 'accidents' still 

 remains a major problem of genetics. 



A general discussion of the effects of captivity on gray 

 Norway rats is deferred until the final report, which will be 

 given when data for the fiftieth generation have been obtained. 



SUMMARY 



The following summary of life processes in gray Norway 

 rats during fourteen years of captivity covers the findings 

 through the twenty-fifth generation. The data analyzed com- 

 prise a total of 8685 litters containing 53,077 individuals. 



In both sexes of gray rats rate and extent of body growth 

 increased gradually as the generations advanced (tables 1 to 3, 

 figs. 1 and 2). At the last generation growth acceleration 



