180 LUTHER BURBANK 



offspring are generally sterile, and the results 

 of the hybridization are not perpetuated. 



Such, then, is the barrier that nature erects, in 

 the interest of race preservation, between species 

 that have already widely diverged. 



But, on the other hand, we have seen many 

 illustrations of the fact that when species a little 

 more closely related are hybridized, the result 

 may be not to produce sterility but to give added 

 virility to the offspring. 



We saw this illustrated, for example, when 

 the walnut of the eastern United States was 

 crossed with the black walnut of California. The 

 hybrid progeny not only exhibited tremendous 

 individual vitality, growing with great rapidity 

 and to enormous size, but they produced an 

 altogether extraordinary abundance of fertile 

 fruit. 



The hybrid variety thus produced named, it 

 will be recalled, the "Royal" constitutes a new 

 race that can more than hold its own against the 

 parent forms. 



And the reason for this seems to be that the 

 two species of walnut had not become sufficiently 

 divergent to introduce a greater diversity of 

 conflicting tendencies than is consonant with 

 racial progress when the strains are brought 

 together. 



