EFFECTS OF INBREEDING ON BODY WEIGHT 7 



Since the change in the food in 1911 and the removal of the 

 colony to new quarters in 1913, the environmental conditions 

 under which the rats were reared have been as uniform as 

 it was possible to make them. All inbred rats, and also the stock 

 animals used for controls, have been subjected to the same con- 

 ditions of light, of temperature, and of nutrition, and they have 

 been cared for in a similar way. Any differences between the 

 two inbred series, or between inbred and stock animals must, 

 therefore, be ascribed to causes inherent in the individuals; 

 they cannot be attributed to the varying action of environment 

 or nutrition. 



2. THE GROWTH IN BODY WEIGHT OF INBRED RATS 



In view of the results that earlier investigators (Crampe, 

 Ritzema-Bos) obtained in their inbreeding experiments with 

 rats, little attention was paid to the fact that the body weights 

 of the animals in the earlier inbred generations were consider- 

 ably less than the norms for stock albino rats of like age. When 

 the individuals of the sixth inbred generation became mature, 

 it was noted that many of them were much larger than stock 

 animals of the same age. This fact was so at variance with the 

 generally accepted belief regarding the effects of close inbreed- 

 ing on body size that it seemed desirable to make a study of the 

 weight increase with age of individuals in the later generations 

 of the two inbred series. 



From the seventh generation on from three to five litters 

 of each inbred series were weighed, first when the animals 

 were thirteen days old, again when they wfere weaned at thirty 

 days of age, and thereafter at intervals of one month until they 

 were fifteen months old. At the thirteen- and thirty-day 

 periods animals of the same sex were weighed together and the 

 average body weight for the group recorded, as at these ages 

 individual differences in body weight are, as a rule, too small to 

 make separate weighings necessary; at all other ages individual 

 records were taken. 



The litters that were used for a study of growth in body 

 weight were all selected at birth on the same basis as the litters 



