32 



BULLETIN 1090, IT. 



S. DEPARTMENT OE AGRICULTURE. 



ing. They mature earlier, produce larger litters, and produce them 

 more regularly than inbreds. Of the young which they produce a 

 much larger percentage are born alive, especially in large litters, and 

 even more of these are raised than in the first generation. Their 

 young show a further increase in birth weight and in later gains. 



Combining the results described in this and in Bulletin No. 1121, 

 there seems no escape from the conclusion that a loss of vigor, especi- 

 ally in fertility, took place as a more or less direct consequence of 

 close inbreeding. The question whether this is an inevitable result 

 of inbreeding, or merely a likely one, as well as other phases of the 

 subject, will be discussed after the presentation of further data. 



APPENDIX. 



Table 6. — Data on the fertility of the inbred stock by years, 1906 to 1920. 



Year. 



Size of litter. 



Mat- 

 ings. 



Num- 

 ber of 

 litters. 



Num- 

 ber of 

 young. 



Aver- 

 age size 

 of litter. 



Litters 

 per 

 year. 



Young 



1 



2 



3 



4 



5 



6 



7 



8 



year. 



1906 



1 



31 



92 



138 



145 



184 



135 



168 



132 



161 



164 



103 



64 



83 



87 



4 

 77 

 173 

 308 

 302 

 303 

 271 

 274 

 234 

 284 

 300 

 196 

 126 

 144 

 208 



13 

 79 

 177 

 275 

 410 

 293 

 276 

 234 

 208 

 288 

 228 

 127 

 89 

 136 

 188 



6 



39 



94 



153 



254 



120 



119 



108 



105 



113 



70 



32 



27 



55 



67 











6.1 

 59.9 

 155.6 

 244.3 

 293.4 

 284.7 

 231.7 

 256.4 

 217.2 

 260.8 

 254.4 

 153.5 

 96.8 

 118.9 

 152.9 



24 

 245 

 598 

 946 

 1,276 

 940 

 853 

 820 

 715 

 872 

 781 

 460 

 309 

 431 

 568 



72 

 679 

 1,673 

 2,573 

 3,863 

 2,359 

 2,256 

 2,043 

 1,835 

 2,181 

 1,824 

 1,015 

 706 

 1,066 

 1,427 



3.00 

 2.77 

 2.80 

 2.72 

 3.03 

 2.51 

 2.64 

 2.49 

 2.57 

 2.50 

 2.34 

 2.21 

 2.28 

 2.47 

 2.51 



3.93 

 4.09 

 3.84 

 3.87 

 4.35 

 3.30 

 3.68 

 3.20 

 3.29 

 3.34 

 3.07 

 3.00 

 3.19 

 3.63 

 3.71 



11.80 



1907 



13 



46 



54 



131 



32 



39 



27 



25 



20 



18 



1 



3 



11 



16 



6 



14 



15 



26 



6 



11 



5 



11 



6 



1 



1 







11.33 



1908 

 1909 

 1910 

 1911 

 1912 

 1913 

 1914 



2 

 2 

 7 

 2 

 2 

 4 



..... 



1 



10.75 

 10.53 

 13.17 

 8.29 

 9.73 

 7.97 

 8.45 



1915 







8.36 



1916 







7.17 



1917 







6.61 



1918 







7.29 



1919 



2 

 2 







8.97 



1920 







9.33 











Table 7. — The percentage born alive. Inbred stock by years, 1906 to 1920. 

 [The average in litters of each size and an index, litters of 1, 2, 3, and 4 weighted 1, 3, 4, and 2, respectively.] 





Size of litter. 







Size of litter. 





Year. 





In- 

 dex. 



Year. 





In- 



























dex. 





1 



2 



3 



4 



5 



6 







1 



2 



3 



4 



5 



6 





1906 



100.0 



100.0 



100.0 



100.0 







100.0 



1914 



81.1 



91.2 



89.6 



82.4 



67.2 



77.3 



87.8 



1907 



87.1 



90.3 



91.6 



90.4 



89.1 



61.1 



90.5 



1915 



86.3 



90.0 



85.2 



79.2 



64.0 



22.2 



85.6 



1908 



85.9 



86.1 



86.8 



84.3 



81.7 



69.0 



86.0 



1916 



79.9 



80.3 



73.8 



51.1 



37.8 



100.0 



71.8 



1909 



89.1 



89.0 



86.7 



80.6 



79.3 



73.3 



86.4 



1917 



83.5 



81.4 



72.4 



60.2 



40.0 



0.0 



73.8 



1910 



85.5 



90.9 



91.6 



85.2 



79.7 



71.2 



89.5 



1918 



82.8 



85.3 



76.0 



71.3 



46.7 





78.5 



1911 



77.2 



84.7 



86.2 



74.6 



53.8 



63.9 



82.5 



1919 



67.5 



88.9 



89.0 



70.4 



50.9 





83.1 



1912 



83.7 



91.3 



86.0 



86.6 



65.1 



51.5 



87.5 



1920 



82.8 



89.9 



82.8 



72.4 



77.5 



41.7 



82.9 



1913 



80.4 



85.9 



90.8 



78.5 



78.5 



63.3 



85.8 



















