168 THE AMERICAN NATURALIST [Vol. LVI 



self-fertilized, the hybrids with their segregating gametes 

 are again crossed and certainly there are more than two 

 factor differences in most of the chromosomes having 

 varying degrees of linkage with each other. At present 

 almost nothing is known about the heredity which the 

 two first generation hybrids may have in common. But 

 all the four homozygous types when crossed singly in the 

 six possible combinations show about an equal amount of 

 heterosis. The double-crossed combination shows no re- 

 duction in vigor of growth, but on the other hand this 

 appreciably increased. This is due, however, in part to 

 a better start as the plants come from large, well-nour- 

 ished seeds grown on vigorous plants, whereas the first 

 cross is handicapped in this respect. 



The doubly hybrid plants are theoretically more di- 

 verse than self-fertilized second geoieration progenies 

 coming from the same parents, but compared with the 

 first and second generations the double cross has features 

 of both. In respect to growth characters the plants are 

 a group of many different first generation hybrids. Very 

 little recombination can take place to allow recessive 

 weaknesses to appear. In fact any recombination that 

 does take place is probably out-balanced by an increase 

 in heterozygosity in other factors. A critical comparison 

 of such double hybrids with their parental first genera- 

 tion hybrids and with their second generation self-fertil- 

 ized sibs in respect to variability of different characters 

 ought to give some indication of the distribution in the 

 chromosomes of the hereditary factors affecting growth. 



In those factors which are independent of the growth 

 of the plant the variability of the double cross should 

 approach or exceed that of self-fertilized second genera- 

 tion. In those characters which are directly dependent 

 upon the vigor of the plant the double cross should re- 

 semble more closely the first hybrid generation. Five 

 characters have been taken and measured in three 

 different but similar lots of plants. These are: number 

 of rows of grain on the ear (pistillate inflorescence) ; 

 nodes of plant, height of plant, length of ear ; and pro- 



