THE COMPOSITION OF A FIELD OF MAIZE. 



By GEORGE H. SHULL, Cold Spring Harbor, N, Y. 



While most of the newer scientific results show the theoretical 

 importance of isolation methods, and practical breeders have dem- 

 onstrated the value of the same in the improvement of many var- 

 ieties, the attempt to employ them in the breeding of Indian corn 

 has met with peculiar difficulties, owing to the fact that self-fer- 

 tilization, or even inbreeding between much wider than individual 

 limits, results in deterioration. 



The cause of such a result is wholly unknown at present. The 

 old hypothesis which sought an explanation of the deleterious 

 effects of inbreeding in the inharmonious or unbalanced constitu- 

 tion produced by the accumulation of disadvantageous individual 

 variations, can hardly stand in the face of the fact that a very large 

 number of plants normally self-fertilize and a noteworthy few have 

 even given up sexual reproduction entirely, without in the least 

 degree lessening their physiological vigor and evident chances of 

 success in competition with sexually produced plants. The dande- 

 lion is propagated parthenogenetically, i. e., its seeds are produced 

 without fertilization, but only the advocate of an unwarrantable 

 theory will maintain that this plant is on that account undergoing 

 a process of deterioration which threatens it with summary ex- 

 tinction. Many species of violets produce most of their seeds 

 from flowers that never open, and one of the most vigorous forms 

 of the small-petaled evening primrose (Oenothera cruciata) does 

 the same. In the breeding of tobacco, it is now well known that 

 cross pollination within the limits of a single strain produces in- 

 ferior offspring and only self-fertilization gives offspring of the 

 highest degree of vigor, though hybrids between distinct strains 

 of tobacco often display a vigor superior to that of either parental 

 strain. Examples could be continued indefinitely, but even one 

 instance in which long-continued inbreeding results in no injurious 

 effects would be sufficient to discredit the old hypothesis. 41 



Some results of the pedigree-breeding of maize at the Cold Springs 

 Harbor Station for Experimental Evolution have suggested a dif- 

 ferent explanation of the deterioration which has been universally 

 observed in self-fertilized maize. For several years a series of 

 investigations on Indian corn has been in progress at the Station, 

 which involved parallel cultures of cross-pollinated and self-fer- 



a For a good discussion of inbreeding see A. D. Shamel, on "The effect of 

 inbreeding 'in plants," Yearbook U. S. D. A., 1905, pp. 377-392. 



