EFFECTS OF INBREEDING ON BODY WEIGHT 91 



in females of the sixteenth to the twenty-fifth generations of 

 the inbred strain closely followed that of the females in the first 

 series of stock controls (compare graph B with graph C in figure 

 8). The animals in both of these latter groups were about 14 

 per cent heavier in adult life than the females in the stock series 

 reared during the past two years (graph D). 



In explanation of the remarkably vigorous growth of the 

 animals in the seventh to the ninth generations of the inbred 

 strain it was suggested in the first paper of this series (King, '18) 

 that: "favorable nutritive conditions following a period of semi- 

 starvation greatly increased metabolic activity and so stimulated 

 the growth impulse that the animals attained an unusually large 

 size. After the maximum effect of the stimulus had passed there 

 was a gradual decline to more normal conditions of metabolism 

 and a corresponding decrease in the average size of the individ- 

 uals," Rats seem to be particularly sensitive to changes in 

 food conditions, more so than is generally supposed, and only 

 by feeding them constantly on a proper diet can their normal 

 weight and fertility be maintained. In light of the valuable 

 researches of McCollum ('18) and his associates, it is evident 

 that the 'scrap' food that the rats received during the period 

 when they exhibited their maximum growth and fertility not 

 only furnished a well-balanced ration as regards the basic food 

 stuffs, but that it also gave a sufficient quantity of the essential 

 accessory foods, 'fat-soluble A' and 'water-soluble B,' to greatly 

 stimulate the growth processes. The experimental diets recently 

 used in our colony have very evidently been deficient in 'fat- 

 soluble A.' As a result the rats have shown marked evidence 

 of malnutrition, although they have received an abundance of 

 food. By rectifying the mistakes of the past and feeding the 

 animals on a properly balanced ration, it is hoped that body 

 growth will again respond to the stimulus of adequate nutrition 

 and that it will be possible to obtain inbred animals that are as 

 large as those in the seventh generation. As after twenty-five 

 generations of brother and sister matings the animals in the in- 

 bred strain were fully as large as were the best stock animals 

 obtainable, it is evident that close inbreeding does not inevitably 



