104 INBREEDING AND OUTBREEDING 



It is more than a mere assumption, then, if a great part of 

 the sterility found by Crampe and Ritzema-Bos is attrib- 

 uted to the same cause. 



The investigations of Weismann and von Guaita are 

 hardly more satisfactory. Weismann inbred a stock of 

 white mice for twenty-nine generations, and found the 

 average number of young for the three 10-gen. periods 

 to be 6.1, 5.6 and 4.2. Where this stock originated, and 

 what method of inbreeding was followed, we are not told. 

 Presumably the gross result was a slight decrease in fer- 

 tility, coincident with the amount of inbreeding, but even 

 this is not certain. As King points out, the average num- 

 ber of litters under observation in the first two genera- 

 tions was twenty-two ; in the last nine generations, three. 

 Clearly there was greater opportunity of selecting 

 healthy breeding stock, as well as a lower probable error, 

 in the earlier part of the experiment, and this might 

 account for the slight difference in fertility found. Von 

 Guaita crossed some of these highly inbred mice with 

 Japanese waltzers and then inbred for six generations. 

 He reports the average number of young in the successive 

 generations as 4.4, 3.0, 3.8, 4.3, 3.2 and 2.3 ; but in view of 

 the vigor almost always expressed in the F^ generation of 

 such crosses one is inclined to doubt the pertinence of 

 these figures to the inbreeding problem. 



Although, as has been pointed out, there is good rea- 

 son for disallowing the claim of these much cited experi- 

 ments on mammals as proofs of the adverse effects of 

 inbreeding through consanguinity alone, there is no inten- 

 tion of denying the isolation of individuals characterized 

 by undesirable qualities from mixed strains by means of 

 Mendelian recombination. Perhaps it is not wise even to 



