186 



E. C. MACDOWELL 



from the back cross. Naturally both the Fi and the back cross 

 offspring occur in greatest numbers about the middle class and 

 decrease in either direction away from this class. But the back 

 cross extends much further than the first generation. It is un- 

 fortunate that the numbers of the Fi are very much smaller 

 than those of the back cross. It may be claimed that with larger 

 numbers more extremes would have been found in the Fi. This 

 is undoubtedly true, but it is highly improbable that any indi- 

 viduals that approached the back cross extremes .would have 



TABLE 1 



Standard deviations of coefficient of size of the first {Fi) and back cross {B.C.) gen- 

 erations in the various families. The numbers of individuals are given for each 

 fraternity (No.). 



1 This family is not from a size cross. Is is used as a check, so is not included 

 in the totals. 



been found. The standard deviations show that there is a real 

 difference in the variability of the two generations. In table 1 

 the standard deviations in terms of the coefficients of size are 

 given for the two generations, one family at a time as well as for 

 the totals of each generation. In every family resulting from a 

 size cross the standard deviation of the back cross exceeds that 

 of the first generation. Family 1493 X 2379 was carried along 

 as a control as the parents were nearly the same size. It will be 

 noted that the standard deviation of the back cross, in this case, 



