358 HELEN DEAN KING 



animals related in various degrees, and they were made as often 

 between parent and offspring as between sibs. Ritzema-Bos 

 states: " Bemerkenswert ist namentlich das Result, dass die 

 Paarung zwischen Geschw^istern viel schlechtere Erfolge lieferte 

 als die Paarung zwischen Mutter und Sohn, resp. Vater und 

 Tochter." Presumably, therefore, my inbred strain, in which 

 all breeding females came from Htters produced by the matings 

 of sibs only, would show an even greater evidence of deterioration 

 in vigor than did the rats inbred by Crampe and by Ritzema-Bos. 

 Data already given show that these inbred rats were much 

 more fertile than stock rats reared under the same environmental 

 conditions, so it is evident that their reproductive vigor was not 

 impaired. In their ability to withstand disease inbred rats com- 

 pared favorably with stock rats. The rat scourge, pneumonia, 

 was quite as prevalent among stock animals as among the inbreds 

 and took its toll of lives as frequently and as quickly in one strain 

 as in the other. Parasitic infection was as common in the stock 

 colony as in the inbred, and severe changes in temperature were 

 followed by just as many deaths among stock animals as occurred 

 in the inbred strain. The rat's power of resistance to disease and 

 to unfavorable environmental conditions did not appear to be 

 lessened by inbreeding under the conditions of these experiments. 

 Records for the growth in body weight of a considerable num- 

 ber of rats belonging in various generations of the two inbred 

 series show the approximate age at which death occurred in all 

 individuals that did not live to the end of the weighing period, 

 which came when the rats were fifteen months old. As similar 

 records were recently obtained for a series of stock animals, it is 

 possible to compare the relative length of life in the two strains 

 and thus to determine whether inbreeding tends to shorten the 

 life of the individuals, as it might be expected to do if it impaired 

 the general vigor of the animals to any extent. 



Table 10 shows the mortality at different ages in such of the A 

 series of inbreds as were used for the determination of the effects 

 of inbreeding on the growth in body weight, given in the first 

 paper of this series. For convenience the data were arranged in 

 generation groups : the last group includes the findings through 



