HYBRID VIGOR OR HETEROSIS 145 



Even Mendel's classic pea hybrids supply further in- 

 stances of increase in size resulting from crossing. Con- 

 cerning them, he says : 



The longer of the two parental stems is usually exceeded by the 

 hybrid, a fact which is possibly only attributable to the greater luxuriance 

 which appears in all parts of the plants when stems of very different 

 lengths are crossed. Thus, for instance, in repeated experiments, stems 

 of 1 foot and of 6 feet in length yielded without exception hybrids which 

 varied in length between 6 feet and 7 1/2 feet. 



.b'ocke, 70 in the book already cited, gives the results of 

 a series of experiments nearly as extensive as those of 

 Gartner and catalogues his own results along with those 

 of his predecessors. The compilation is so careful, so 

 painstaking, and so complete that one may turn to the 

 nnal conclusions of the author without fear of error as 

 far as the facts are concerned. He says: " Crosses be- 

 tween different races and different varieties are distin- 

 guished from individuals of the pure type, as a rule, by 

 their vegetative vigor. Hybrids between markedly dif- 

 ferent species are frequently quite delicate, especially 

 when young, so that the seedlings are difficult to raise. 

 Hybrids between species or between races that are more 

 nearly related are, as a rule, however, uncommonly tall 

 and robust, as is shown by their size, rapidity of growth, 

 earliness of flowering, abundance of blossoms, long dura- 

 tion of life, ease of asexual propagation, increased size of 

 individual organs, and similar characters." 



The attention of these earlier hybridizers was mainly 

 directed towards interspecific crosses, but they also noted 

 a great number of instances in which crosses between 

 closely related forms, such as varieties or sub-varieties 

 of cultivated plants, gave remarkable increments in 

 10 



