RULES AND RATIOS OF INHERITANCE 229 



(6) But careful study, when large numbers of individu- 

 als are available, shows that the F2 generation is much more 

 variable than the Fi generation. It reveals also the pres- 

 ence of a few extreme individuals, differing little or not at 

 all from the two parental types. 



This greater variability in F2 as compared with Fi, in 

 cases in which the parents differ quantitatively, has been 

 found to occur in great numbers of cases, of which the 

 following may be mentioned: size and shape in gourds, 

 squashes, beans (Emerson 19 10) ; length of the ears in 

 maize (East and Hayes 191 1) ; color, size of seeds and of 

 petals in flax (Tammes 191 1); time of flowering in peas 

 and in cotton (Tschermak 1911); size in ducks (Phillips 

 1912); winter hardiness in cereals (Nilsson-Ehle 1912); 

 skin color in negro-white crosses (Davenport 19 13); size 

 in rabbits (MacDowell 19 14) ; and in many other cases. 

 There appears to be no known case of "blending inheri- 

 tance" in which the F2 generation is not more variable 

 than the Fi generation, so that it appears clear that in all 

 such cases the inheritance is based on differences in a num- 

 ber of genes ("multiple factors"), each of which has a 

 quantitative effect on the characteristic. 



(7) The later generations, produced from the inter- 

 breeding of the F2 individuals and their descendants, show 

 much the same relations as are found in F2. Most of them 

 are intermediate, though with a number of different grades. 

 A few are extreme; these however are so rare that they 

 are observed only when large numbers are bred. 



(8) The intermediate individuals in F2 or later genera- 

 tions, when interbred or self-fertilized, usually do not breed 

 true; they produce a number of different grades, so that 

 their offspring are variable. This is because the intermedi- 

 ate individuals (a) include a number of different grades, 

 with different constitutions; also (b) many of them are 

 heterozygotic. The commonest intermediates are those with 



