EFFECTS OF INBREEDING ON BODY WEIGHT 101 



weights seemed to decrease with the advance of inbreeding, but 

 in the later generations the variabihty was greatly influenced 

 by environmental and nutritive conditions. Until these latter 

 factors can be controlled, it will not be possible to draw any 

 definite conclusions regarding the effects of inbreeding per se 

 on the variability in body weights. 



SUMMARY 



1. The data given in the present paper show the growth and 

 variability in the body weights of 296 males and of 310 females 

 belonging in the sixteenth to the twenty-fifth generations of two 

 series (A and B) of albino rats that were inbred, brother and 

 sister from the same litter. 



2. Owing to economic conditions, many of these rats were 

 not reared under very favorable conditions of environment and 

 of nutrition, and in consequence they did not grow as rapidly 

 nor did they attain as great a maximum body weight as did the 

 individuals in the earlier generations of this inbred strain. 



3. In every generation from the sixteenth to the twenty-fifth 

 the males were heavier than the females at all age periods after 

 thirty days (tables 1 to 4). This result agrees with the finding 

 for the inbred rats of the earlier generations, and also with that 

 for various series of stock Albinos. Apparently, therefore, long- 

 continued inbreeding has not changed the normal body-weight 

 relations of the sexes at any age period for which records have 

 been taken. 



4. In the A series of inbreds the rate and extent of growth in 

 body weight were much the same as those in the B series of 

 inbreds : in the adult animals there was a difference of only about 

 2 per cent in the average body weights of corresponding groups 

 of males and females in the two series (tables 8 and 9 ; fig. 5 and 6) . 



5. Close inbreeding for twenty-five generations has not altered 

 the form of the growth graph for the albino rat to any extent. 



6. Rats belonging to the later generations of the inbred strain 

 were not as heavy at any age period as were the animals in the 

 earlier generations, but they were much superior in body weight 

 to stock Albinos reared under similar conditions of environment 

 and of nutrition (figs. 7 and 8). 



