204 Plant-Breeding 



in practice we shall probably find as many exceptions to 

 the formula? as confirmations of them, even though the 

 exceptions can be explained, after we find them, by Men- 

 del's principles of heredity. 



The probable limits of mendelism in the production of 

 new varieties. It has been said that we shall soon be 

 able, as a result of Mendel's discoveries, to predict varie- 

 ties in plant-breeding. Before considering this question, 

 we must recall the fact that a cultural variety is a succes- 

 sion of plants with characters sufficiently marked and 

 uniform to make it worth while cultivating in place of 

 some older variety. Now and then it may be worth 

 while to introduce some new energy or new trend into a 

 general lot of offspring by making wholesale crosses, not 

 expecting ever ta segregate any particular variety or 

 strain from the progeny ; but these cases are rare, and the 

 gain is indefinite *and temporary. So far as our knowledge 

 at present goes, we see no warrant for the hope that we can 

 predict varieties with any degree of exactness, at least 

 not beyond a very narrow effort. Following are some of 

 the reasons that seem to argue against the probability 

 of useful prophecy of varieties so far as the mende- 

 lian results are concerned : (1) We do not know what 

 plants will mendelize until we try. (2) Even in plants 

 that do not mendelize, one-half of the offspring have 

 stable characters. But we cannot predict for even this 

 half, for it is impossible to determine beforehand which 

 seeds showing dominant characters (and these are three- 

 fourths of the offspring) will "come true." Dominance, 

 as we have seen, is of two kinds in respect to its behavior 

 in the next generation, constant and hybrid ; and the 



