232 THE BREEDING OF ANIMALS 



216. Researches of Ritzema Bos. An interesting 

 experiment with in-breeding white rats for thirty genera- 

 tions is reported by Ritzema Bos. 1 Beginning with an 

 albino female mated with a wild rat, the first mating re- 

 sulted in twelve offspring. Seven of this litter were 

 bred .to a white male but unrelated. The resulting off- 

 spring were closely in-bred for six years. The matings 

 were brothers to sisters and parents to offspring. The 

 average size of the litters for twenty generations and 

 covering a period of four years remained practically 

 constant. The last ten generations born during the 

 last two years of the experiment showed a decided decrease 

 in the fertility of the matings. The average size of 

 litters by years is shown in the table. 



DECREASED FERTILITY DUE TO IN-BREEDING. (Bos) 



1887 1888 1889 1890 1891 1892 



71 71 7H 6H 4& 3t 



Not only the average size of the litters decreased in 

 thirty generations from 7J to 3 J, but the number of mat- 

 ings which were sterile increased greatly from the first 

 to the last generations, as shown in the following table : 



MATINGS WHICH PROVED STERILE. (Bos) 



1887 1888 1889 1890 1891 1892 



2.63 5.55 17.39 50 41.18 



There is evidence also in this experiment that the 

 constitutional vigor of the offspring of parents which 

 were the result of long-continued in-breeding was materi- 

 ally injured. The time of the appearance of weakness 



1 Ritzema Bos, "Biol. Centralb.," XIV, 1894. See also Mor- 

 gan, "Experimental Zoology," p. 188. 



