EFFECTS OF INBREEDING ON BODY WEIGHT 6 



in the body weight of rats belonging to fifteen inbred genera- 

 tions; subsequent papers will deal with the effects of inbreeding 

 on fertihty, on constitutional vigor and on the sex ratio. To 

 avoid needless repetition, a complete bibliography will be given 

 in the final paper. 



I wish to express my great obligations to the Director of The 

 Wistar Institute, Dr. M. J. Greenman, for placing at my dis- 

 posal ample facilities for carrying on the investigation and for 

 his unfailing interest in the problem. I am much indebted, 

 also, to Dr. H. H. Donaldson for advice and criticism that have 

 been most helpful when difficult situations have arisen at various 

 stages of the work. 



1. MATERIAL AND OUTLINE OF THE EXPERIMENT 



The albino rat (Mus norvegicus albinus) is a domesticated 

 variety that is well adapted to confinement and is easily cared 

 for. It breeds throughout the year, producing litters of rela- 

 tively large size, and under proper environmental conditions 

 from three to four generations can be obtained in a single year. 

 This combination of characteristics makes the rat very useful 

 as a laboratory mammal, and exceptionally favorable material 

 for a study of the problem of inbreeding. 



The experiments were started with a litter containing four 

 rats, two males and two females, which was born on the 10th of 

 May, 1909. This litter was selected from a number of new-born 

 litters in the general stock colony merely because it happened to 

 contain the number of individuals required for starting the in- 

 vestigation, and not because of the ancestry, the size, or the 

 vigor of the individuals. One of the females in the litter was 

 called 'A,' and her descendants form the 'A series' of inbreds; 

 the other female was designated as 'B,' and her descendants 

 form the 'B series' of inbreds. 



The plan of the experiment required that females A and B, 

 as well as all of the females subsequently used for breeding, 

 should be mated twice with a brother from the same litter and 

 then twice with an unrelated stock male. Since the mating of 



