364 HELEN DEAN KING 



piled show, as does the investigation of Pearson et al. ('03), that 

 the duration of Ufe in women is longer than it is in men and that 

 women are the less susceptible to disease at all ages. 



The various physical defects, so prevalent among Crampe's 

 ('83) inbred rats, were all found among my inbred rats at the 

 beginning of these experiments, but they were due to malnutri- 

 tion, not to inbreeding, since they entirely disappeared when the 

 animals received proper food. Among the thousands of inbred 

 animals that were reared during the past five years some few, not 

 to exceed a dozen in all, lacked one or both eyeballs. This defect 

 has also appeared, at times, in stock animals. On the average, 

 one in every 10,000 rats born in the stock colony is tailless. This 

 abnormality, as Conrow ('15, '17) has shown, involves the skeletal 

 structure in the entire pelvic region. The inbred colony has con- 

 tained only one tailless individual as yet. Unfortunately, this 

 I at was destroyed by the mother soon after birth, so it was not 

 carefully examined. Neither of these defects appears to be 

 heritable, and neither can be due to inbreeding, since each has 

 appeared also in a stock that is outbred. No other abnormalities 

 of any kind have appeared in the animals of the inbred strain up 

 to the present time when the individuals of the twenty-eighth 

 generation are approaching maturity. The findings in this series 

 of experiments, therefore, do not give support to Ritzema-Bos' 

 contention that inbreeding tends to cause ''eine grossere Pradis- 

 position fiir Krankheiten und das Entstehen von Missbildungen." 

 When a considerable number of animals belonging to any series 

 exhibits various kinds of malformations, it is safe to assume that 

 either environmental and nutritive conditions are unfavorable to 

 normal development, as in the early part of the present series of 

 experiments, or that there is an inherent weakness in the stock 

 used that is brought out and accentuated by random inbreeding, 

 as seemed to be the case with Crampe's rats. 



No data are available for a direct comparison between stock 

 and inbred rats as regards their relative activity at different ages, 

 but several series of experiments have been made in different 

 psychological laboratories in which the behavior of rats from this 

 inbred strain was compared with that of stock controls. 



