EXPERIMENTAL. 



33 



STATISTICAL TREATMENT. 

 Coefficients of Variability Litter by Litter. 

 Coefficients of variability were calculated for each measurement in 

 nearly all litters. (See table 4.) Being based on small numbers of obser- 

 vations, these coefficients show no uniformity, but they indicate that the 

 different parts measured vary to some extent independently of each other. 

 In some measurements a greater variability is found in a back-cross litter 

 than in the Fi generation in the same family, whereas in other measure- 

 ments from the same sets of animals the Fi generation may show greater 

 variability; for instance, in family 647 the coefficient of variability for 



P. O. in generation Fi is 5.1, while in the back-cross litters it is 3.8, 2.7, 

 and 3.4; in the same family the coefficients of variability for Te. are: 

 Fi = 1.3, back-cross = 3.5, 3.6, and 2.8. Similarly the measurements Na. 

 and H. show coefficients for Fi larger than any for the back-crosses, and 

 Za., N., M., U., F., T. have coefficients of variability for F x smaller than 

 any for the back-crosses. The coefficients of Fi for the other ten meas- 

 urements are within the range of the back-cross coefficients. The other 

 families all show similar irregularities. 



