14 SIZE INHERITANCE IN RABBITS. 



SIZE DIFFERENCES. 



From characters of form we come to those of size. The possible 

 application to size of this theory of multiple factors so strongly indi- 

 cated in color was recognized by Nilsson-Ehle (1907). He found that 

 crosses of wheat and oats with long and short stalks gave intermediate 

 hybrids whose offspring ranged all the way between the grandparents. 

 The length of the head of wheat acted in crosses as though it were 

 dependent upon a strong dominant-shortening factor and two similar 

 weaker factors for length that lack dominance. For a normal head 

 crossed with a compactum type (a special short head) gave a simple 

 monohybrid ratio in F 2 of 3 compactum to 1 non-compactum. Yet a 

 normal crossed with a very long type of head gave no grouping into 

 two classes in F 2 , but a continuous series of grades between the two 

 grandparents, forming a frequency curve whose mode was intermediate. 



Spillman (1902) has described fourteen crosses between wheats hav- 

 ing long and short heads. FI gave intermediate heads. In F 2 "the 

 progeny of each plant of the previous generation presented every gra- 

 dation between the parents, forming a continuous series which, in 

 most cases, extended beyond both parents." Crosses between strains 

 of maize differing in the height of stalk have been made by Emerson 

 (1910). Tall (225 cm.) X short (90 cm.) gave a uniform F! with a 

 mean height of 182 cm. F 2 gave plants as small as the small parent 

 and others taller than the first generation, but none as tall as the tall 

 parent. One case is given (Emerson, 1911) of a cross between Missouri 

 dent (tall) and a dwarf Tom Thumb pop, from which the F 2 genera- 

 tion had plants fully as tall and as short as the parents. 



East and Hayes (1911) have described three crosses between varieties 

 of maize with long and short ears which showed marked increased varia- 

 bility in the second generation. The distributions of the second gen- 

 erations include ears that are beyond the means of the parental races, 

 whereas the FI ears (though large on account of increased vigor from 

 the cross) do not approach the small parent. The standard deviations 

 and coefficients of variability show this increased variability in every 

 case. Crosses between races differing in the number of rows per ear 

 were also made by East and Hayes; these gave increased variability in 

 F 2 . Five crosses gave similar results. A cross between two 12-rowed 

 lines gave no increased variability in F 2 . Similar results were found 

 by Shull (1910 and 191 la) in comparable crosses. Two nearly pure 

 selfed families had average variabilities of 9.08 and 9.06 per cent. 

 The second generation of the cross had a variability of 12.63 per cent. 

 This increase was said to be due to the existence of genotypic differences 

 which appeared through the segregation of the different characters into 

 different germ cells. This case is exactly like East's; and, although 

 Shull does not mention Nilsson-Ehle in this connection, he has given 

 good evidence for the belief in the existence of several factors that 

 determine the number of rows. 



