Hybrid Vi^or 159 



that an inbred race should be homozyjjjous; therefore all 

 the indi\'i(luals involved would ha\e the same f^erminal 

 constitution. A cross between any two such individuals 

 would really not be effective in producing a hybrid, 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- 

 fertilized families results in a progeny of as great vigor, 

 size, and productiveness as are possessed by families 

 that 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 continu- 

 ously, but when crossed with another race it immediately 

 seems to regain all the original vigor. It is as though all 

 germ plasm contains the potentiality of developing vig- 

 orous individuals. This potentiality, however, cannot 

 express itself until the proper combination 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. 

 When reciprocal crosses are equal it suggests a Mendclian 

 phenomenon. Is it possible that hybrid vigor may be 

 explained in terms of Mendelism ? 



These are five "laws" of hybrid vigor presented by 

 Shull, in 1910. 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 Siiui.i. in connection with 

 hybrid vigor is of interest. Ciranted that hybrid vigor is an estab- 

 Ushcd fact, the question of its practical use in connection with 

 crop plants should be taken into account. If a farmer after years 



