BEGINNINGS OF THE HETEROSIS CONCEPT 15 



tional hybridizations, and it is easy to recognize heterosis as a potent factor 

 in the remarkable values displayed by many of these new varieties. But 

 while Burbank made great use of hybridizations in his plant breeding work, 

 he did not recognize hybridization, as such, as the source of the large size 

 and remarkable vigor of his new varieties. For him the role of hybridization, 

 aside from the bringing together of desirable qualities possessed separately 

 by the two chosen parents, was merely the "breaking of the types." In this 

 way the variability in subsequent generations was greatly increased, thus 

 enlarging the range of forms from among which to select the most desirable 

 for recognition as New Creations. 



There are many other important observations and philosophical considera- 

 tions that bear a close relationship to our current understanding of heterosis, 

 and which antedated the recognition of heterosis. It would take us too far 

 afield, however, to discuss these related observations at length. We can 

 make only this passing reference to the highly significant work of Charles 

 Darwin in demonstrating that cross-fertilization results, in many cases, in 

 increased size, vigor, and productiveness as compared with self-fertilization 

 or with other close inbreeding within the same species. 



Darwin did not recognize this increased vigor as identical with hybrid 

 vigor, nor specifically attribute it to the differences between the uniting 

 gametes. To him it only demonstrated a method which would inevitably 

 preserve by natural selection any variation that might occur — whether me- 

 chanical or physiological — which would make cross-fertilization more likely 

 or even an obligate method of reproduction. With heterosis established as a 

 recognized pattern of behavior, or type of explanation, we can now interpret 

 Darwin's demonstrated superiority of crossbreds as examples of the occur- 

 rence of heterosis. We may go even further and include the whole field of 

 sexual reproduction in showing the advantages of heterosis. These result 

 from the union of two cells — the egg and the sperm — extremely difTerentiated 

 physiologically, and in all dioecious organisms also dififerentiated genetically. 



Let us briefly consider several investigations which foreshadowed the 

 procedures now used in growing hybrid corn — for somewhere in the course 

 of this work with corn the heterosis principle was first definitely recognized. 



Two techniques are characteristically associated with the work of the 

 "hybrid-corn makers." Uncritical commentators have mistakenly considered 

 these techniques synonymous with the development of the hybrid-corn pro- 

 gram itself. These are (a) cross-pollination by interplanting two different 

 lines or varieties, and the detasseling of one of these lines which then sup- 

 plies the seed to be planted; and {b) controlled self-pollination. 



In deciding what part these two methods played in the develoi)mcnt of the 

 heterosis concept, we must first consider why these methods were used by 

 various workers and how their use affected the experimental conclusions. 



Dr. William J. Beal, of Michigan Agricultural College, apparently was 



