EFFECTS OF INBREEDING ON BODY WEIGHT 39 



At the thirty day period only was the coefficient for the female 

 group significantly greater than the corresponding one for the 

 male group. The greater variability in the body weights of 

 the females at this age is doubtless correlated with the fact 

 that during early postnatal life female rats are growing more 

 rapidly than the males. From the evidence at hand it appears 

 that inbred males are more variable in body weight than inbred 

 females. Jackson ('13) and King ('15 a) have already noted that 

 in gi'oups of stock albinos the males tend to be more variable in 

 body weight than the females at corresponding age periods. 



As growth records were taken for only a comparatively small 

 number of animals in each generation of the two inbred series, it 

 did not seem advisable to calculate the coefficients of variation 

 for each generation separately, since Pearson has shown that with 

 numbers less than twenty-five the empirical standard deviation 

 is usually too small. The combined records for the individuals 

 of the two series (A, B), divided into three groups as shown in 

 table 10, were used in calculating the coefficients of variation for 

 the body weights of the generation groups as given in table 16. 



As table 16 shows, correspondmg coefficients for the three gen- 

 eration groups varied considerably in some cases, but there was 

 a decided tendency in Jaoth sexes for all the coefficients to become 

 smaller as the inbred generation advanced. The difference be- 

 tween the average coefficients for the males and for the females 

 in successive generation groups was about two points in each 

 instance. It appears, therefore, that variability in body weight 

 diminished at a fairly uniform rate from one inbred generation 

 to the next. The average decrease for each generation was com- 

 paratively slight, amounting to less than one per cent, and it 

 was about the same for the two sexes, although in all generation 

 groups the males tended to be somewhat more variable in body 

 weight than the females. 



The question arises as to how the variability in the body 

 weight of the inbred rats compares with that in stock animals in 

 which there is no inbreeding. Obviously in this instance one 

 should compare inbred and stock animals taken from the same 

 strain and reared under similar environmental conditions, since, 



