EFFECTS OF INBREEDING ON BODY WEIGHT 27 



The graphs in figure 9 run close together throughout their 

 entire course, but the graph for the B series is higher at all 

 points than that for the A series. It appears, therefore, that 

 growth in body weight was somewhat more vigorous in the fe- 

 males of the B series than in those of the A series. 



It seems a rather significant fact that in both figure 8 and 

 figure 9 the graphs run parallel from the beginning until the end 

 of their course; they do not cross and recross at various points, 

 as one might expect would be the case with graphs for two 

 series of rats from the same ancestral stock that were reared 

 simultaneously under the same environmental conditions. Fe- 

 male K and her mate, the ancestors of the B series of inbreds, 

 were heavier animals at the time that they were killed than 

 were the progenitors of the A series of inbreds, female A and her 

 mate. Body weight in the rat is so dependent on physical 

 condition, however, that a single weighing of the animals when 

 they were at an advanced age would not necessarily give a true 

 idea of the relative size of the animals at an earlier age period. 

 The difference in the size of the two pairs of rats with which the 

 experiments were started, together with the fact that after the 

 sixth generation the descendants of female B were relatively 

 heavier animals than the descendants of female A, point to the 

 conclusion that the difference in the size of the animals in the 

 two series was not due to chance or to environment, but that it 

 was dependent in some way upon the inheritance of genetic 

 factors for growth. 



Table 13 gives the body weight data for the total of 156 males 

 and 169 females in the seventh to the fifteenth inbred generations 

 for which weight records were taken (a combination of the data 

 in table 11 and in table 12). 



Table 13 brings out one fact of interest: the average body 

 weight of the male inbred rats increased with age up to the end 

 of the weighing period when it w^as 358.7 grams; the average 

 body weight of the females was at its maximum at the 425 day 

 period, and then fell off sUghtly at the final weighing. Indi- 

 vidual rats show a pronounced difference as regards the time 

 that they attain their maximum body weight and, as a rule, the 



