Hybrid Vigor 177 



ever, Jones's explanation seems distinctly more serviceable than 

 any other that has been offered. 



Skull's scheme to take practical advantage of hybrid vigor 

 in such a plant as corn has one distinct drawback. As was brought 

 out in chapter ii, the size of a corn plant is limited by the size of 

 the seed which produces it, and this, in turn, is limited by the size 

 of the mother-plant upon which the seed developed. Since 

 Shull's races A and B were both inbred races, they must have been 

 rather small. Consequently, whichever wa>' the cross was made, 

 the seeds containing the Fi embr>'os would be limited in size by the 

 small size of the female parent, so that the P', plants would get a 

 poor start and would never be able to attain the size that would 

 have been possible had they come from large seeds. 



Jones (9) suggests a way of overcoming this dilTiculty. 

 Starting with four pure races, cross .1 with B and C with I). This 

 will result in two vigorous Fi types, either one capable of develop- 

 ing large seeds. Cross these two, and u double hybrid results 

 which combines, in good part, the advantages of all four of the 

 original races, and is not limited in size by starting from a small 

 seed. Of course there is a certain drawback here also, since the 

 generation used for the crop is an F2 with respect to the two original 

 crosses, and has somewhat less than the maximum vigoi on that 

 account. The relative advantages and disadvantages of such 

 breeding plans can be evaluated only by experiment. Junes 

 claims to have gotten better results from his double cross method 

 than could be obtained from Shull's plan. 



From the discussion that has been preseiUeci in lliis 

 chapter, one may safely conclude that the ])henomena 

 which arise in connection with inhreedin*^ and outbreed- 

 ing can be explained satisfactorily in terms of the Men- 

 dehan mechanism of inheritance. It should l)e recog- 

 nized that inbreeding is not injurious per se (^throuL^di mere 

 fact of consangtiinity), but because it serves to isohite 

 undesirable recessive tyi)es from a liybrid nu'xture. 



The proof on this point is that inl)rec'diiig in homozy- 

 gous stock results in no deterioration, joxios (9) has 



