38 SIZE INHERITANCE IN RABBITS. 



class. This tendency of F x offspring to exceed the midparental measure- 

 ments (class 9) may readily be interpreted as the effect of increased vigor 

 in consequence of the cross. It does not persist in the back cross, or in 

 F 2 . East (1909) and Shull (1909) have shown that hybridization may 

 cause an increased stimulus to cell division, which has no relation to 

 inheritance. The classic work of Darwin (1876), that showed the 

 increased size of crossed over self-fertilized plants, may also be cited. 



Perhaps the most surprising fact shown by the distributions of the 

 first generation is the occurrence of individuals, some of whose parts 

 are almost as small as those of their small parent and others with 

 measurements larger than those of their large parent. This does not 

 mean that any Fi animal was, as a whole, as extreme as its parents. 

 A marked difference in variability is seen when curves for different char- 

 acters are compared. When the range is wide it is as much extended 

 in one direction as in the other. Such a difference is found when the 

 curve for the ulna (U.), which has a very wide range, is compared with 

 the curve of the skull length (0. M.), which has a very limited range; 

 yet these curves are based on the very same individuals. With narrow 

 ranges comparable to that of the skull length are the curves for the 

 two partial skull lengths (P. O. and S. M.) and the mandible measure- 

 ments (Mi and M 2 ) ; but the width at the teeth (Te.) and the anterior 

 zygomatic width have wider ranges. The posterior width of the 

 zygoma (Zp.), the length of the nasal bone (Na.), and the four leg 

 bones (H., XL, F., T.) all show still wider ranges. 



Pearson (1902, p. 378) found similar inequalities in the different 

 parts of the human body : 



"The inheritance of all characters does not appear to be the same. The 

 inheritance of forearm is for all four cases sensibly less than the inheritance of 

 span, and that of span less than that of stature." 



Hatai (1907) found greater variability in the length of the nasal bone 

 and the zygomatic width in skulls of albino rats than in any other skull 

 characters. 



The above discussion on the ranges in different characters based on the 

 curves of the first generation applies equally well to the curves of the back- 

 crosses. If the offspring from a back-cross with the father occupied 

 the middle class between the midparental for Fi and the class for the 

 father, they would fall in class 5. In four measurements (see table 0) 

 the means fall approximately in class 5, namely, the skull length meas- 

 urements (P. O. and S. M.), the mandible measurement (M 2 ), and the 

 ulna measurement (U.) . Of the remaining measurements, five lie nearer 

 class 6 than class 5 — that is, exceed expectation by one class — while 

 four lie nearer class 4 or lower classes with an average deviation of about 

 two classes. The deviations are pretty evenly divided between plus 

 and minus with reference to class 5. This is what would be expected 

 on a theory of blending inheritance, but also on a theory of Mendelian 



