INBREEDING EXPERIMENTS 113 



per day, rate of growth, and variability. The proper in- 

 terpretation of their results is somewhat obscure, unless 

 one postulates the origin of frequent mutations. The 

 number of generations bred and the number of families 

 under observation were not sufficient to demonstrate the 

 segregation of differences in these characteristics, though 

 this is to be expected since these qualities are sympto- 

 matic of genera], vigor and general vigor was increased 

 by crossing. The difficulty, however, lies in the fact that 

 continued parthenogenetic multiplication which is possible 

 in Hydatina had the same effect as continued inbreeding. 

 Shull introduces the interesting speculation that this sim- 

 ilarity is due to a gradual adjustment of nucleus to cyto- 

 plasm during the asexual propagation this being as- 

 sumed to bring about the same results as a gradual 

 approach toward homozygosis. We are inclined to at- 

 tribute both changes to environmental causes, believing 

 that if a proper change in diet had been made vigor would 

 have been maintained. 



While we are not justified in concluding from these 

 experiments that inbreeding accompanied by rigid selec- 

 tion will be beneficial to bisexual animals, they certainly 

 show close mating is not invariably injurious. They in- 

 dicate that the results of inbreeding depend more upon 

 the genetic composition of the individuals subjected to 

 inbreeding rather than upon any pernicious influence in- 

 herent in the process itself ; and, as will be emphasized 

 more strongly later, it is a wholly different matter 

 whether inbreeding results injuriously through the inheri- 

 tance received, or whether consanguinity itself is respon- 

 sible. Yet such a status for the problem is unsatisfactory. 

 The experiments on animals bring to light no facts which 



