EFFECTS OF INBREEDING ON BODY WEIGHT 51 



there was, in every case, a difference of about two points between 

 the coefficients for successive generation groups. In these in- 

 bred rats, therefore, variabiUty in body weight did not de- 

 crease proportionately as the homozygosis of the individuals 

 increased. The results are complicated by the fact that part, 

 at least, of the variability in body weight that was measured 

 by the coefficients of variability was due to environmental 

 action and cannot be distinguished from the variability due to 

 genetic growth factors. When the data for a number of later 

 inbred generations have been examined it may then be possible 

 to ascertain the probable extent of variability due to environ- 

 ment and to correlate the amount of homozygosis in the indi- 

 viduals with the extent of their variability in body weight. 



The records, as given above, show unquestionably that the 

 variability in body weights of the individuals decreased as the 

 inbreeding advanced. The results, therefore, do not accord with 

 Walton's ('15) theory that continued inbreeding tends to increase 

 rather than to diminish variability. In an able criticism of this 

 theory Castle ('16 c) says: ''It is difficult to understand how on 

 any theory of heredity inbreeding could be expected to increase 



variability within a single inbred line On a Mende- 



lian theory it would be expected that inbreeding,* brother with 

 sister, for a large number of generations would result in the pro- 

 duction of a number of homyzygous lines, each of which by it- 

 self would be entirely devoid of variability, except that due to 

 environmental agencies." The results so far obtained in this 

 inbreeding experiment with the rat are in harmony with Castle's 

 view, though in such a complex organism as the rat it is not 

 probable that any degree of inbreeding will produce lines that are 

 "entirely devoid of variability, except that due to environmental 

 agencies." 



In discussing the effects of close inbreeding on Drosophila, 

 Castle et al. ('06) state: "These experiments show no appreciable 

 effect of inbreeding. In every case the brood reared under the 

 best and most uniform conditions has the highest average num- 

 ber of teeth (on the sex comb) , irrespective of whether or not in- 

 bred. The same may be said of variation in size. Inbreeding 



