EXAMPLES OF MULTIPLE FACTORS. 15 



Emerson (1910) reported three crosses between beans of different 

 sizes in which an increased variability was found in F 2 . The coefficients 

 of variability in F 2 (based on sizes, weights, and shapes) are " usually 

 twice and in some cases six or seven times as great as in Fi" (p.744). 

 East and Hayes (1911) and Emerson (1910) have given cases of in- 

 creased variability in F 2 from crosses between strains of maize differing 

 in the size of grain. Crosses between strains of Linum which differed 

 in the size of seed were studied by Tammes (1911). Length and width 

 were investigated separately. Both showed constancy in an interme- 

 diate Fi and variability in F 2 . The range of variation transgressed the 

 ranges of the parents, though the parental extremes were equaled only 

 in rare cases. It was estimated (on the basis of the frequency of appar- 

 ently pure parental forms in F 2 ) that four or possibly five similar inde- 

 pendent Mendelizing units allelomorphic to their absence would explain 

 the difference in variability in the first and second generations. Tam- 

 mes (1911) found similar results in crosses between lines differing in the 

 lengths and widths of petals. 



The first experimental evidence of the appearance of increased varia- 

 bility in the size of animals after crosses is given in a preliminary report 

 by Phillips (1912). Rouen ducks, whose average weights were, males 

 2,321 grams, females 2,244 grams, were crossed with mallards whose 

 averages were, males 1,068 grams and females 928 grams. The Fi 

 average for 10 males was 1,665 grams; for 3 females, 1,587 grams; 33 

 F 2 ducks had averages a little higher. The coefficients of variability 

 for the two generations were : 



Fi females =3.43; males=5.32. Fi females =11. 07; males= 12.57 



When classified in an absolute scale a similar increase in range was 

 easily observed in F 2 . 



STRUCTURAL CHARACTERS. 



Several structural characters have been found that seem to depend 

 on two or more similar units. Of these the most important is the ligula 

 in oats. Nilsson-Ehle (1909) made five crosses between varieties with 

 and without liguhe. In F 2 the following ratios were found; 4:1, 19:1, 

 44:1, 71:1, 274:1. The entire second generation of the cross that 

 gave 19:1 was carried on to the next generation. The results found 

 from this accorded very closely with the expectation, based on the 

 assumption that the second generation ratio was due to the presence 

 of two factors, each able to produce ligula). The higher ratios were 

 believed to be due to three and four factors. In discussing this case 

 Nilsson-Ehle says (1909, p. 85) : 



"Dieser Fall is also insofern von besonderem Interesse, als es durch densel- 

 ben gelungen ist, das Prinzip von der Anwesenheit und Abwesenheit der 

 Elementareigenschaften als Glieder je eines Merkmalspaares de facto auf ein 

 Gebiet zu. iibertragen, wo man sonst nur per analogiam oder aus indirekten 

 Griinden zur Annahme dieses Grundprinzips gelangen wiirde. Mit grosserem 



