182 ■ E. C. MACDOWELL 



ferent strains of cotton of equal height Balls ('07) found a great 

 variability in the heights of the second generation. The first 

 generation was unifornly close to 100 cm., the second generation 

 ranged from 30 cm. to 100 cm. Hayes ('12) has crossed varieties 

 of tobacco with similar numbers of leaves which produced plants 

 with the same number of leaves in the first generation. The leaf 

 number for the plants of the second generation was very variable. 

 Keeble and Pellew ('10) have worked out a very clear explanation 

 of an increase in variability found in a second generation follow- 

 ing the crossing of two semi-dwarf races of peas. The parental 

 races were very constantly between 3 and 4 feet tall. The first 

 generation showed increased vigor, but a constant height of 7 to 

 8 feet. The second generation ranged from 1| to 8 feet. This 

 case differs from those preceding in that the factors, to whose 

 recombination the new grades in the second generation were due, 

 did not produce the same effects. One factor increased the dis- 

 tance between the nodes, the other increased the thickness of the 

 stem, and this thickening enabled the internode to attain greater 

 length. One parental race had the factors for thick stems and 

 short internodes, the other had the factors for thin stems and 

 long internodes. The recombinations in the second generation 

 produced "plants with long internodes and thick stems (8 feet as 

 well as plants with short internodes and thin stems (1| feet). 

 The combinations found in the parents also occurred. More- 

 over, the ratios in the four classes closely approximated expec- 

 tation. In this case height is dependent upon two Mendelizing 

 factors. 



In other cases size may depend upon a single factor. Some of 

 these cases are mentioned : tall and dwarf peas, Mendel ; tall un- 

 branched habit vs. dwarf branched sweet peas, Bateson and Pun- 

 nett ('08); axial vs. terminal position of bean pods, Emerson 

 ('04); tall vs. dwarf Antirrhinum, Baur ('11); dwarf vs. normal 

 tomatoes, Drinkard ('08); long vs. short styles of Oenothera, de 

 Vries ('01, p. 435); long vs. short wings of Drosophila, Morgan 

 ('11) ; long vs. short hair in various mammals, Castle ('05) ; brach- 

 ydactylous digits in man, Farabee ('05) and Drinkwater ('08); 



