42 



BULLETIN 1090, U. S. DEPARTMENT OF AGRICULTURE. 



malities. At the most it merely brings out more frequently variations 

 which depend on recessive factors, which factors are either already 

 in the stock or which arise from time to time as mutations. 



DIFFERENTIATION IN VIGOR. 



These results raise the question as to whether the inbred families 

 have become differentiated in a similar manner in the less tangible 

 traits connected with vigor, such as fertility, rate of growth, and 

 ability to raise the young successfully. 



The characters of this kind which have been used in the present 

 work were defined in the first part of this bulletin. Their relations to 

 each other and their variation under the influence of external con- 

 ditions were discussed there at some length. Under the heading of 

 f ertility are considered the average size of litter, the number of Utters 

 produced per year by mature parents, and the product of these 

 factors, the number of young produced per year. The rate of growth 

 is studied in the birth weight, in the gain between birth and weaning 

 at 33 days of age, and in the sum of these weights, the weight at 33 

 days. The percentage of the young born alive, the percentage of 

 these raised to weaning, and the product, the percentage of all of the 

 young raised to weaning, measure the vitality of the young and the 

 capacity of the females for raising them. 



Averages for each of these characters have been calculated in each 

 inbred family for two periods, 1906 to 1910, and 1911 to 1915. In 

 the former period most of the families were composed of several lines 

 of descent, connected with each other only in the first generation of 

 inbreeding. Many of the lines had been eliminated before the 

 second period, in which, therefore, the families might be expected to 

 be more homogeneous. The figures on mortality among the young 

 and also the weights were calculated separately for each size of litter. 

 Indexes were obtained, as previously, by weighting the averages for 

 litters of 1, 2, 3, and 4, in the ratio 1:3:4:2. The results are given in 

 Tables 7 to 15. 



Examination of the tables reveals considerable differences be- 

 tween the families. Table 1 shows the extremes in the period from 

 1911 to 1915. 



Table 1. — Families ivith extreme records in the period from 1911 to 1915. 



Character. 



Best family. 



Poorest family. 



Size of litter 



Litters per year 



Young per year 



Percentage born alive 



Percentage raised of those hern alive 



Percentage raised 



Birth weight of young raised 



Gain from birth to 33 davs 



Weight at 33 days 



No. 11(2.90) 



No. 23 (3.80) 



No. 35(10.34) 



No. 39 (91.5 percent). 

 No. 2 (89.3 percent).. 

 No. 20 (78.5 percent). 

 No* 13 (91.5 grams).... 

 No. 11 f 169.1 grams)... 

 No. 11 (260.2 grams)... 



No. 1 (1.74). 

 No. 38 (2.46). 

 No. 1 (4.68). 

 No. 3 (76.5 per cent). 

 No. 3 (76.1 percent). 

 No. 3 (57.7 percent). 

 No. 2 (75.6 grams). 

 No. 2 (122.2 grams). 

 No. 2 (197.8 grams). 



