1 26 THE AMERICAN NATURALIST [Vol. LV 



There are a number of instances of coherence or link- 

 age knowTi in maize, but if the characters studied to dato 

 are a fair sample the role of linkage must be of minor 

 importance. Linkage relations have not been studied in 

 sufficient detail even to state with assurance that the 

 characters are arranged in linear series corresponding 

 to the number of chromosomes, although this conclusion 

 is indicated. The linkages of most of the Mendelian 

 characters are very loose and it would seem necessary to 

 conclude that, if the characters of maize are arranged in 

 a linear series, the chromosomes must be either very 

 long or very flexible. 



While admitting that linkage would meet the objec- 

 tions urged against the simple hypothesis that the sup- 

 pression of recessive characters explains heterosis, it 

 may be well first to make sure that any such assumption 

 is necessar^^ An examination of the maize literature 

 indicates that the difficulty of securing uniform strains 

 with the vigor of the first generation has been assumed 

 rather than demonstrated. No case was found where 

 selection following hybridization had been continued long 

 enough to approximate homozygosity. There are also 

 very few cases where the more vigorous F2 individuals 

 have been chosen as parents of the Fg. The most exten- 

 sive series of experiments are those of Emerson and 

 East (1913). 



Height is probably the most satisfactory character to 

 use as a measure of heterosis. There are 23 compari- 

 sons of Fi and F., populations in the work of Emerson 

 and East. To these six can be added from our own ex- 

 periments. 



In these 29 cases the mean of the Fg was below that 

 of the Fj in every instance but in ten of the 29 cases the 

 largest of the F2 plants equalled or exceeded the largest 

 of the Fj individuals and in every case where a progeny 

 was growTi from a plant near the upper limit of the 

 range of the Fj its mean exceeded that of the Fj. 



Other characters reported by Emerson and East for 



