HYBRID VIGOR OR HETEROSIS 143 



experiment, a striking instance of the stimulating effects of crossing the 

 breeds; for the smallest variety, whose height rarely exceeded two feet, 

 was increased to six feet, whilst the height of the large and luxuriant 

 kind was very little diminished. 



It is evident that in this particular case Knight was 

 dealing with dwarf and standard peas, and dominance of 

 the tall standard habit of growth is to be expected. This 

 is not the correct interpretation of the majority of his ob- 

 servations on hybrid vigor, however ; a sufficient number 

 of really striking manifestations of the phenomenon were 

 found to give adequate foundation for his anti-inbreeding 

 principle, elaborated by Darwin fifty years later. 



Probably the most extensive series of early experi- 

 ments on hybridization were those of Gartner. 74 This 

 enthusiastic worker crossed, or attempted to cross, every- 

 thing available to him. According to Lindley, 128 he made 

 10,000 pollinations between 700 species, and produced 250 

 different hybrids. Many of his attempted crosses either 

 failed to produce seed, or if seed was produced, gave feeble 

 plants; but a great number of the hybrids, where the 

 crosses were made between plants not too distantly re- 

 lated, showed distinct evidence of hybrid vigor mani- 

 fested in many different ways. Gartner speaks especially 

 of their general vegetative luxuriance, increase in root 

 development, height, number of flowers, the facility of 

 their vegetative propagation, their hardiness and early 

 and prolonged blooming. He says : 



One of the most conspicuous and common characteristics of plant 

 hybrids is the luxuriance of all their parts, a luxuriance that is shown in 

 the rankness of their growth and a prodigal development of root shoots, 

 branches, leaves, and blossoms that could not be induced in the parent 

 stocks by the most careful cultivation. The hybrids usually reach the 

 full development of their parts only when planted in the open, as 



