102 INBREEDING AND OUTBREEDING 



young, obtained by crossing an albino female with a white 

 and gray male. These animals were inbred in various 

 degrees for seventeen generations. During the experi- 

 ment many rats showed great susceptibility to disease, 

 divers kinds of abnormalities, diminished fertility, and 

 increased total sterility. 



Similarly Ritzema-Bos started his investigations with 

 a litter of twelve rats obtained by crossing, this time an 

 albino female with a wild Norway male. This stock was 

 inbred in different ways for six years, during which time 

 he claimed to have obtained about thirty generations. His 

 results did not corroborate those of Crampe in so far as 

 susceptibility to disease or appearance of malformations 

 are concerned, but there was a gradual decrease in size 

 of litter and a gradual increase in percentage of infertile 

 matings, as is shown in the following table : 



Year of inbreeding 1 23456 



Ave. number in litter 7.5 7.1 7.1 6.5 4.2 3.2 



Per cent, infertile 



inatings ".0.0 2.6 5.6 17.4 50.0 41.2 



These investigations, in spite of the habit biologists 

 have of citing them, are not calculated to settle the ques- 

 tion they undertook to answer. Ritzema-Bos himself 

 criticizes those of Crampe, because he believes them to 

 have been started with a weak strain. Miss King, how- 

 ever, thinks the weakness of these rats, as indicated by 

 their susceptibility to disease, the appearance of mal- 

 formations, and their tendency to sterility, was due to 

 the conditions under which they were kept. She had a 

 similar experience during the earlier part of her own 

 experiments, and found that inadequate nourishment was 

 largely the cause. But it is not for this reason that we 



