4 HELEN DEAN KING 



brother and sister from the same Utter is the closest form of 

 inbreeding possible in mammals, no other form of mating has 

 ever been used to obtain inbred litters. In every generation 

 all females used for breeding have belonged to inbred litters; 

 none of them have ever been taken from 'half-inbred' litters 

 obtained by the mating of inbred females with stock males. 

 In the early part of the experiment the number of breeding 

 animals was, of necessity, small. In every generation after the 

 sixth about twenty females from each series were used for 

 breeding, so that approximately 1000 young were obtained in 

 each generation. 



All four of the rats used in starting the experiment were 

 killed when they were no longer wanted for breeding purposes. 

 Each rat was weighed, measured, and carefully dissected. When 

 the various records were compared with the norms for the albino 

 rat (Donaldson, '15) it was found that all of the rats were under 

 the average body weight for their age, but that they were nor- 

 mal in all other respects as far as could be determined by the 

 usual methods of laboratory procedure. The fact that these 

 individuals were sound, healthy animals and normal in all 

 essential respects is a point on which I wish to place special 

 emphasis in order to forestall the possible criticism that the re- 

 sults obtained in this work were due to the use of an exceptional 

 strain of rats. 



In the earlier generations the inbred rats exhibited all of the 

 defects which are popularly supposed to appear in any closely 

 inbred stock. Many females in both series were sterile, and 

 those that did breed usually produced only one or two litters 

 which were generally of small size. A considerable proportion 

 of the rats were dwarfed, or stunted in their growth, and many 

 of them developed malformations, particularly deformed teeth. 

 The animals showed, also, a steady decline in vitality in suc- 

 ceeding generations and usually died at a relatively early age. 

 If the experiments had been discontinued at this point the re- 

 sults would have been a confirmation of the conclusion reached 

 by Darwin and by several other investigators, that inbreeding 

 invariably leads to sterility and to physical degeneration. 



