If, however, cross-fertilization occurs with a different 

 strain, an immediate increase of vigor follows. Shull's 

 results were as follows: 



Average yield per acre of two pure strains . . 29 bushels 



Yield of first cross 68 bushels. 



Yield of original strain (cross-fertilized for 5 years) 61| 

 bushels. 



Here the problem is to determine the real reason for the 

 increased vigor in the first cross of two separate strains. 



Additional examples of the increased vigor of hybrids 

 over the parent forms are Burbank's hybrid walnut (Cali- 

 fornia black walnut x English walnut) which grows larger 

 and much faster than either parent, the hybrid dewberry 

 (Western dewberry x Siberian raspberry) which ripens 

 earlier and is larger and more productive than either parent. 



As examples of the increased size of hybrids among our 

 domestic animals we may note that crosses between Che- 

 viots and Leicesters are larger than either parent, and such 

 crossings are much practised when lambs and sheep are 

 reared for the market (See also Chapter 17 on In-breeding) ; 

 that the mule is extensively bred in the United States on 

 account of its hardiness in extreme climatic conditions, its 

 longevity , its relative freedom from disease and injury, and 

 its ability to live on coarse food; and that the first crosses 

 between Poland-China and Chester-white swine, and be- 

 tween Shorthorn and Aberdeen-Angus cattle are popular 

 as feeders. 



The work of Webber and Swingle with citrus fruits is 

 also worthy of mention. By crossing the ordinary orange 

 with the hardy trifoliate orange, a useless variety, they se- 

 cured among the hybrids several that combined quality 

 with frost resistance. These hybrids are often called "cit- 

 ranges. " The Rusk, the Willets and the Morton are three of 

 the new fruits. As a result oranges can be grown 400 miles 

 noFth of their present range, and able to endure a tempera- 

 ture of 8° F. 



Another hybrid the "tangelo" resulted fron crossing 

 the tangerine with grape fruit. 



The investigations by D. F. Jones showed that grains 

 of hybrids weighed more than those of the self-fertilized 

 forms. 



Shull has attempted to explain hybrid vigor or heterosis 

 by the theory of heterozygosis, i.e. that hybrid vigor is in 

 proportion to the number of factors in which the parents 



79 



