28 



BULLETIN 1090, U. S. DEPARTMENT OF AGRICULTURE. 



level of the normally bred stock in the years preceding 1911 in which 

 their records show an actual rise, while they were holding their own 

 genetically in the subsequent years in which their records show a 

 marked decline. The situation brings out most strikingly the diffi- 

 culty in drawing conclusions from results obtained in different years. 

 The figures which show the changes in the percentage of young 

 raised in the different sizes of litter separately (Tables 8 and 17) 



1906 1906 19/0 /9J2 

 /4 



19/4 



J9I6 1916 /920 



1 



/2 



to 



e 



6 



t 



I 



« 1 • — 



\ I 



\ I 



s »* * 



* ■ m* 1 » 



**'' \ « 



\ / 



\ / 



% t 



^ , 



At. » »* * • 



/ \ %.---»» % f 



/ \ \ * ^^^ 



/ \ * ' ^^ 



B 



Fig. 10.— The average number of young produced per year by mature matings. Inbreds (A ) and controls 



(B), 1906 to 1920. 



bring out an interesting point, which was touched on earlier in this 

 bulletin (page 15) . It will be seen that there was not much difference 

 in the percentages raised in litters of 1 to 5 in the early years. In 

 litters of 1 and 2 there has not been much subsequent decline and the 

 controls do not have much advantage over the inbreds. In litters 

 of 3 the controls begin to show a distinct superiority, which increases 

 in litters of 4 and 5, the decline among the inbreds becoming rapid. 

 Thus it is in the ability to raise large litters that the inbreds show 

 their deterioration most strikingly. 



