Application of Biological Principles to Plant Breeding 117 



Since all commercial methods of selection in maize, as 

 well as other naturally cross-bred species, have as their 

 ultimate goal the isolation of good homozygous strains (for 

 this is what the words "selection to type" mean), it is quite 

 evident that the longer selection has been carried on the 

 more of this stimulus due to heterozygosity or hybridity 

 is lost. No method of breeding naturally cross-bred species 

 therefore, where size and total yield are the main objects, 

 is proper unless these facts are taken into consideration 

 and the methods so modified as to utilize them. This is 

 done by growing only the first hybrid generation of crosses 

 between good strains. Nor is it alone in wind-pollinated 

 field crops, such as maize, that these methods are useful. 

 Horticultural crops such as tomatoes and eggplants can be 

 grown from hand-hybridized seed with a profit greatly 

 exceeding the extra cost of its production. It may be that 

 even certain trees can be hybridized to advantage for 

 undoubtedly Burbank's quick-growing walnuts are due to 

 this phenomenon. Furthermore, it accounts for the fact 

 that all asexually propagated crops worthy commercial 

 supremacy, such as grapes and potatoes where yield is the 

 object of prime importance, are always hybrids. Their 

 mode of commercial propagation is such that the first 

 hybrid generation can be indefinitely prolonged. 



It is not easy to leave this subject without mentioning 

 the important role which this growth stimulus due to 

 hybridity may have played in the evolution of the higher 

 plants. In self-fertilized species, for example the violets, 

 the fact that the hybrid between two nearly related strains 

 was more vigorous than either parent type would have 

 given it such an advantage in the ordinary struggles for 

 existence against inhospitable environment, that the chances 

 are greatly in favor of its surviving to produce recombi- 



