494 THE AMERICAN NATURALIST [Vol. LII 



eration parental and hybrid progenies were grown in 

 1915. Seeds produced by flowers which were selfed on 

 certain individuals in these progenies furnished the sec- 

 ond generation, which was grown in 1916. Flowers on 

 selected plants in the second generation parental and F 2 

 hybrid progenies were again selfed to furnish the third 

 generation parental and the F 3 hybrid progenies, which 

 were grown in 1917. 



Some of the flowers on Fj hybrid plants in 1915 were 

 pollinated from plants in the first generation progenies 

 of the original parent plants of either variety. From 

 the resulting seed % Pima and % Gila back-crosses were 

 grown in 1916. Plants were selected in each of the % 

 back-cross progenies because of their approach to the 

 corresponding predominant parent in respect to impor- 

 tant characters. The Pima back-cross plants were pol- 

 linated from a plant in the second selfed generation of 

 the Pima parent and the Gila back-cross plants were pol- 

 linated from a similar Gila individual. The resulting 

 % Pima and % Gila back-cross progenies were grown 

 in 1917. 



Every effort was made to grow the various parental 

 and hybrid progenies under as nearly as possible uni- 

 form conditions in respect to soil, irrigation and cul- 

 tural treatment. All comparisons of hybrids and parents 

 have been made on the basis of progenies grown the 

 same season, in order to obviate the influence of different 

 weather conditions. Measurements on the different 

 plants were made, as far as practicable, upon organs 

 which occurred at the same nodes of the axis and 

 branches and which were in the same stage of develop- 

 ment. The number of plants on which most of the char- 

 acters were measured in each generation were, in round 

 numbers : 



