188 E. C. MACDOWELL 



In general the size of all the measurements of the first genera- 

 tion is plainly intermediate, although the modes and means of 

 the various frequencies are markedly above the midparental. 

 This high distribution may readily be interpreted as the effect 

 of increased vigor from the cross. Similar plant crosses show a 

 like increase in size in the first generation (see East '09, Shull 

 '09, Darwin '76). A marked difference in variability is seen in 

 these first generation frequencies when different characters are 

 compared. When the range is wide it is as much extended in 

 one direction as the other, showing that the extreme lows are 

 not to be accounted for by underdevelopment, as an asymmetrical 

 variation might suggest. Such a difference is found when the 

 distribution for the ulna, which has a wide range, is compared 

 with the distribution of the skull length, which has a very limited 

 range (table 2). Four of the remaining measurements are sim- 

 ilar to the skull length; two are intermediate; six are wide, slightly 

 less than the ulna. Yet these frequencies include the same in- 

 dividuals. Pearson ('02) found similar inequalities in different 

 parts of the human body. Hatai ('07) found greater variability 

 in the length of the nasal bone and in the zygomatic width in 

 skulls of albino rats than in any other skull characters. 



The variations in the ranges of distributions of the different 

 characters, described for the first generation, are also found in 

 the back cross. The means of the back cross fall near the class 

 half way between the midparental (middle class) and the small 

 parent to which the first generation females were back crossed. 

 A very marked feature of these back cross frequencies is the 

 number of measurements that are as low and lower than the 

 small parent, and as high as, and higher than the mode of the 

 first generation frequencies in which their mothers are included. 

 Since the father and grandfather in all the families was the same 

 rabbit, the actual values of the male (small) parental class in all 

 families is the same, and the values of the adjacent classes differ 

 very slightly in the different families. This means that all 

 animals falling in classes near his class have very nearly the 

 same sizes in the different families. This makes the occurrence 

 of these short measurements the more, convincing evidence of 

 segregation. 



