SIZE INHERITANCE 197 



A considerable number of cases of size inheritance has now 

 been studied in both animals and plants. Their results may 

 be summarized thus: (1) When animals or plants are crossed 

 which have racial differences in size or other characters, in 

 respect to which each race shows continuous variation about 

 a different mean, the Fi progeny are of intermediate size.^ 

 They may or may not be more variable than the races 

 crossed, but quite commonly are not. (2) The F2 generation 

 as a whole commonly varies about the same intermediate 

 mean as the Fi generation, but its variability as measured by 

 the standard deviation or the coefficient of variation is usu- 

 ally greater than that of the Fi generation. The increased 

 variability of F 2 as compared with Fi may in extreme cases 

 include forms larger than the larger parental race or smaller 

 than the smaller race, and which show a tendency to vary 

 in F3 about the same size as characterized the F2 parent. 



Some illustrative cases may be cited. Phillips (1912, 1914) 

 crossed two breeds of ducks which differed markedly in size, 

 namely Rouens and Mallards. The average adult weight of 

 the Rouen race used w^as, for males, two thousand three hun- 

 dred grams, and for females two thousand two hundred and 

 thirty-seven grams. Corresponding weights for the Mallard 

 race were one thousand sixty-eight and nine hundred and 

 twenty-eight grams respectively. The Rouens accordingly 

 were more than tw^ice as large as the Mallards. The two 

 races did not overlap in weight, as appears from Table 29, 

 where the animals are classified by weight. In this table the 

 mean weight of the Mallards is taken as the center of class 

 2 and the mean weight of the Rouens as the center of class 10. 

 Each sex was classified separately but the two are combined 

 in classes bearing the same class number in Table 29. The 

 seventy Fi offspring have their mode in the intermediate 



1 I leave out of consideration here such differences as exist between till ami dwarf 

 peas, and between brachydactylous and normal men. In such cases a simple 

 Mendelizing difference exists, which shows both dominance and segregation in 

 typical fashion. Aside from this simple difference, however, ordinary size differ- 

 ences exist in such cases, which I doubt not follow the ordinary rules of size in- 

 heritance. 



