GEORGE HARRISON SHULL 



Princefon University 



Chapter 2 



Beginnings of 

 the Heterosis Concept 



The heterosis concept was first definitely recognized in the work with hybrid 

 corn. Before attempting to define this concept, however, we will take a brief 

 look at some of the observations of early workers which indicated the prob- 

 able presence of heterosis, and where recognition of heterosis as an important 

 biological principle might have been expected. 



The first hybridizer of plants, Dr. J. G. Koelreuter, noted some impres- 

 sive examples of excessive luxuriance in his Nicotiana hybrids. These were 

 isolated observations which suggested no theory as to why these hybrids 

 should exceed their parents in size and general vigor. Koelreuter cannot be 

 said to have had a heterosis concept. Probably every conscious producer of 

 hybrids since Koelreuter's time has made similar observations of the exces- 

 sive vigor of some hybrids over their parents, so that such hybrid vigor has 

 ceased to cause surprise. But the general acceptance of hybrid vigor as a nor- 

 mal phenomenon did not estabUsh a heterosis concept. It was merely the 

 summational effect of oft-repeated experience. 



Thomas Andrew Knight noted the deterioration of some of the old stand- 

 ard horticultural varieties, and concluded that such varieties have a natural 

 life-span and gradually decline as the result of advancing senility. He saw 

 that such decline makes it necessary to develop new varieties which will start 

 off with the vigor of youth. Although Knight himself produced many such 

 new varieties, some of which were produced by hybridization, it is not ap- 

 parent that he thought of hybridization as an agency for the production of 

 such new vigor. Although he advanced a theory concerning physiological 

 vigor and its decline, he did not recognize the heterosis concept. 



Luther Burbank also produced numerous varieties, often following inten- 



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