EFFECTS OF INBREEDING ON BODY WEIGHT 97 



of a single generation was only about one-third of that used for 

 a group of three generations. 



The average coefficients for the three groups of females com- 

 prising the animals in the sixteenth to the twenty-fourth gener- 

 ations of the inbred strain are all lower than those for the corre- 

 sponding groups of males (table 13), and they also fail to show a 

 significant decrease in size as inbreeding advanced. The average 

 coefficient for the body weights of the females in the twenty-fifth 

 generation is considerably smaller than that for any of the three 

 generation groups, but here also no definite conclusion seems 

 warranted, since the small number of records on which the 

 coefficients are based may be responsible in great measure for 

 the result. 



The animals in the seventh to the fifteenth generations of the 

 inbred strain lived under environmental and nutritive conditions 

 that were fairly uniform and seemingly very favorable to growth 

 and to fertility. The body weights of these individuals showed 

 a slow decrease in variability with the advance of inbreeding, as 

 the relative size of their coefficients of variability indicates (King, 

 '18; table 16). During early life the rats in the sixteenth and 

 seventeenth generations lived under the same environmental and 

 nutritive conditions as the animals of the preceding generations, 

 and at this time they were all seemingly somewhat less variable 

 in body weight than were the individuals in the fifteenth gener- 

 ation. Before the weight records for these rats were completed, 

 a change in diet became necessary, as 'scrap' food of the required 

 quality and quantity could no longer be obtained. The effects 

 of the change in food became very apparent in the course of a 

 few weeks, and, as individual rats responded differently to the 

 altered conditions of nutrition, there was a marked increase in 

 the variability of the body weights in the animals of all ages. 

 When the coefficients of variabiHty were calculated from the 

 series of body-weight data obtained for the animals in the six- 

 teenth to the eighteenth generations, they were found to be 

 somewhat larger than those for the animals in the fifteenth 

 generation, as was expected from the observed appearance of 

 the animals. The animals in the later generations of the inbred 



