EXPERIMENTAL. 



39 



inheritance involving multiple factors without dominance. However, 

 a very marked feature of the frequencies of the back-cross offspring; is 

 the number of measurements that are as low as and lower than the 

 small parent and as high as and higher than the modes of the Fj curves 

 in which their mothers are recorded. Since the father and grandfather 

 in all these families were the same rabbit, the actual values of classes 

 near his class in different families are very nearly the same. All ani- 

 mals that fall in his class have the same size as he, and those that fall 

 in an adjacent class are of nearly the same absolute size as compared 

 with each other. This makes the occurrence of these short measurements 

 the more convincing evidence of segregation. The standard deviations 

 of the FI and back-cross generations given in table 6 are further evidence 

 of segregation, in that they show that in general the variability of the back 

 cross is greater than that of FI while taking into account the differences 

 in numbers in the two generations. In six measurements (O. M., S. M., 

 Zp., T., MI., M 2 ) the standard deviation of the back-cross offspring is 

 significantly greater; in three measurements (Na., U., F.,) it is signifi- 

 cantly less. In the four other measurements (Za., P. O., H., T.) the 

 difference lies within the probable error and so is not significant. 



TABLE 6. Means and standard deviations of the. FI and back-cross generations, based 

 on the classifications given in table 7 and expressed in terms of those classes. 



SUMMARY. 



Studies of bone measurements have shown that there is greater 

 variability in the second generation from a size cross than in the first. 

 This is shown by the use of coefficients of size in which the different 

 measurements for one animal are expressed, as well as by the classi- 

 fication of the two generations (one character at a time) in relation to 

 their parents. Numerous individuals are found in back crosses with 

 measurements as short or shorter than the corresponding measurements 

 in the small parent; others are found with parts larger than the corre- 

 sponding modes of the first generation. The standard deviations of 

 the grand totals of these classifications indicate an increase in the back 

 cross in about half of the measurements. 



