EFFECTS OF INBREEDING ON BODY WEIGHT 45 



were not due to inbreeding but solely to malnutrition. There is 

 a possibility, therefore, that many of the bad results obtained 

 in other inbreeding experiments with rodents may have been 

 produced, in part at least, by unfavorable environmental or 

 nutritive conditions. The published records of the former work 

 give no details regarding the manner in which the experiments 

 were conducted, consequently there is no way of determining 

 to what extent external conditions were responsible for the out- 

 come. Both Crampe and Ritzema-Bos worked with hybrids, 

 which frequently exhibit a tendency to sterility as others have 

 noted, and the animals were inbred promiscuously for the most 

 part. Even the most favorable environmental conditions could 

 not be expected to keep such animals up to normal standards for 

 body size or for fertility. 



In the present series of experiments improper feeding through 

 four successive generations did not permanently impair the growth 

 power of the individuals, which responded at once to the stimu- 

 lus of a well balanced diet. The rats in the fifth and those in 

 the sixth inbred generations grew much more vigorously than 

 their forefathers, and many of them attained an adult size equal 

 to that which is normal for the albino rat. 



The maximum effect of the stimulus given to the growth im- 

 pulse by adequate nourishment did not seem to be reached 

 until the seventh generation when some of the animals were 

 larger than any albino rats as yet recorded. In the two fol- 

 lowing generations the average body weight of both males and 

 females at various age periods decreased slightly, but they were 

 still far above the norms for stock animals and higher than 

 the averages for the individuals in the eleventh and succeeding 

 generations. From the tenth to the fifteenth generations there 

 was no very marked change in the average body weights of the 

 rats at corresponding age periods, but the body weights tended 

 to decrease slightly as the inbreeding advanced. 



It seems probable that the exceptionally vigorous growth of 

 the rats in the seventh to the ninth inbred generations was wholly, 

 or in great part, a response of the organisms to very favorable 

 nutritive conditioais following a period of partial starvation. 



