IN-BREEDING 233 



in the offspring was coextensive with the decline in 

 fertility. 



The comparative mortality of the young increased 

 rapidly, as shown in the following table : 



INCREASED RATE OF MORTALITY DUE TO IN-BREEDING. (Bos) 



1887 1888 1889 1890 1891 1892 



Per cent Per cent Per cent Per cent Per cent Per cent 



3.9 4.4 5.0 36.7 36.4 45.5 



The bad effects from in-breeding were more noticeable 

 when brother and sister were mated than when parents 

 were mated with offspring. Of the matings between 

 parent and offspring 21.4 per cent were sterile, while 

 those between brother and sister were 36 per cent infertile. 

 A tendency to decrease in size is indicated by the record 

 of weights of different generations. Full-grown male 

 rats of the first generation weighed 300 grams each. In 

 the tenth generation the weight had decreased to 275 

 grains and at the end of six years the weight of rats had 

 declined to 240 grams. 



217. The Wistar Institute experiments. Extensive 

 in-breeding experiments by Helen Dean King l at the 

 Wistar Institute, Philadelphia, with white rats seem to 

 have resulted in disproving the theory that in-breeding 

 is always and necessarily followed by evil results. In 

 this investigation white rats have been as closely in-bred 

 as possible for twenty-two generations. More than 

 10,000 in-bred rats have been observed during a period 

 of seven years. The original stock was two pairs of 

 albino rats. Each pair was used as the foundation for a 

 series of in-breeding experiments. In each generation 



1 Journal of Heredity, vol. 7 (1916), p. 70. 



