INBREEDING EXPERIMENTS 103 



feel that both of these experiments should be disregarded. 

 Each ivas started with hybrid stock, and such experi- 

 ments with hybrid stock bring in an additioyial compli- 

 cation, Mendelian recombination. The only type of 

 investigation on bisexual animals calculated to offer criti- 

 cal evidence on the effect of inbreeding per se must be 

 carried on with stock which has already been inbred long 

 enough to reduce the genetic constitution of the animals 

 to an approximately homozygous condition. Then, and 

 then only, can the effect of more extended inbreeding be 

 determined without confusion as to the interpretation 

 of the results. 



Miss King has pointed out a part of the difTiculties 

 involved in starting with a hybrid stock. In one of her 

 experiments the progeny of a cross between a wild Nor- 

 way rat and an albino was inbred for several generations. 

 She found that while the majority of the F-^ females were 

 fertile, at least 25 per cent, of the Fo females were com- 

 pletely sterile and 10 per cent, of those which did breed 

 cast only one or two litters. In the strains extracted from 

 this cross there was variation in degree of fertility, but 

 none was found which exhibited the high degree of fer- 

 tility usually existent in the albino rat. No endeavor to 

 select fertile strains was made and one cannot say whether 

 or not rigid selection would have isolated them, but the 

 researches of Detlefsen^' on hybrids of the genus Cavia 

 to which the common guinea-pig belongs indicate this to 

 be a probability. These investigations as well as those of 

 Easf^^ on the genus Nicotiana show conclusively that 

 various hereditary factors are involved in the partial 

 sterility exhibited in many species crosses, and that tliese 

 factors may be expected to recombine in the usual manner. 



