EFFECTS OF INBREEDING ON BODY WEIGHT 99 



effects of inbreeding; nor can it be ascribed to a difference in 

 the genetic constitution of the two series of animals, since no 

 new 'blood' was introduced into the general stock colony from 

 1913 to 1917. From the evidence given, one seems warranted 

 in assuming that the marked difference in the variability of the 

 two series of stock animals was due, in great part, to the effects 

 of changed conditions of nutrition which so greatly influenced 

 the body growth of the individuals in the second series. It 

 is probable also that the extremes of temperature to which many 

 of these rats were subjected also affected their variability in 

 body weight to some extent, although the effects of temperature 

 changes were very much less than those of nutrition. 



Since the variability in the body weights of outbred stock 

 Albinos was seemingly greatly affected by nutritive and environ- 

 mental factors, one would naturally conclude that these factors 

 would likewise influence the variability in the body weights of 

 inbred animals reared simultaneously with and under the same 

 conditions as the stock Albinos. The increased variabihty in 

 the inbred animals of the sixteenth to the twenty-fifth gener- 

 ations is, on this assumption, the result of environmental and 

 nutritive action, and it cannot be cited in support of Walton's 

 ('15) contention that continued inbreeding tends to increase 

 variability. It is interesting to note in this connection that a 

 comparison between the average coefficients for various groups 

 of inbred rats and those for stock Albinos indicates that changed 

 conditions of nutrition produced a much greater effect on the 

 variability in the body weights of stock Albinos than it did on 

 that of the animals in the later generations of the inbred strain. 



In this experiment, owing to the action of environment and 

 of nutrition, it is impossible to determine the changes, if any, 

 that inbreeding per se produced on the variability in the body 

 weights of the animals in the later generations of the inbred 

 strain. This study of variability is of value, therefore, mainly 

 because it shows that in the later generations of inbreds there 

 existed between the two series (A and B), and between the two 

 sexes, the same relative variability in body weights as that found 

 in the earher generations. Twenty-five generations of brother 



