Hybrid Vigor i6i 



produce a hybrid in any sense, so that it would not be 

 surprising that such a cross fails to bring hybrid vigor. 



4. "A cross between plants belonging to two self- 

 fertihzed families results in a progeny of as great vigor, 

 size, and productiveness as are possessed by famihes 

 which have never been self-fertilized." The conclusion 

 from this is that inbreeding results in no permanent 

 loss of vigor. A race may "run out" if inbred con- 

 tinuously, but when crossed with another race it im- 

 mediately seems to regain all the original vigor. It 

 is as though all germ plasm contains the potentiality 

 of developing vigorous individuals. This potentiality, 

 however, caimot express itself until the proper combina- 

 tion of conditions arises, and this proper combination 

 seems to be connected in some way with hybridizing. 



5. "Reciprocal crosses between two distinct self-] 

 fertilized families are equal" in producing hybrid vigor, i 

 When reciprocal crosses are equal it immediately sug- 

 gests Mendelian segregation. Is it possible that hybrid 

 vigor may be explained in terms of Mendelism ? These 

 are the five "laws" of hybrid vigor developed by Shull 

 iniQ 10. It should be noted that they are not hypotheses 

 but observed facts. The hypotheses were developed 

 later when more of the facts were in. 



A practical suggestion made by Shull in cormection 

 with hybrid vigor is of interest. Granted that hybrid 

 vigor is an estabUshed fact, the question of its practical 

 use in connection with crop plants should be taken into 

 account. If a farmer after years of work has finally 

 developed a desirable new strain of corn by selection, or 

 isolation, or both, he is not likely to favor hybridizing 

 with some other strain in any wholesale way. He must 



