Application of Biological Principles to Plant Breeding 115 



per cent taller than the average of the parents. The 

 hybrids also have somewhat larger leaves than the average 

 of the two parents although the number is generally inter- 

 mediate. This statement is true in general for crosses 

 between the varieties of most other species. When true 

 species are crossed, however, the behavior of the first hybrid 

 generation is somewhat different. Sometimes there is a 

 great increase in vigor. N. tabacum X N. syhestris gives 

 hybrids that are nearly double the average height of the 

 parents. N. tabacum X N. alata, on the other hand, gives 

 hybrids that are less than one-fourth the size of the smaller 

 parent. Both of these crosses are sterile, so that the differ- 

 ence in behavior cannot be correlated with sterility. One 

 can simply say that species vary in their affinity to cross 

 with other species. All gradations are found, from those 

 that produce full quotas of viable seed, to those where only 

 an occasional seed is found or where the capsule simply 

 develops without the formation of seed. And in general 

 the additional vigor of the first hybrid generation increases 

 with the ease of making the cross. With perfect ease of 

 crossing the stimulus is roughly a function either of the 

 number or of the kind of character pairs for which the 

 individual is heterozygous. 



Species naturally self-fertilized like tobacco or wheat 

 must get along without the increased vigor due to hetero- 

 zygosis. One notices the difference only when artificial 

 crosses are made. Species which in nature are cross- 

 fertilized, however, are usually heterozygous for so 

 many characters that one does not think of their vigor as 

 being largely due to this cause. The fact is only brought 

 to notice when the species is self-fertilized artificially, 

 for this tends to isolate homozygous strains (Fig. 49). 



