Neo-Mendelism 83 



plant breeders for some years. The races of plants 

 improved by artificial selection have usually reverted 

 to type when selection ceases. This fact was recognized 

 for a long time, but was first pointed out clearly by DE 

 VRIES (i). Since then we have always expected this 

 result, that no improvement will maintain itself but 

 will run back unless the selection is continuous. When 

 a practical breeder announces that he has developed 

 by selection a new race which continues to breed true* 

 without further selection we are inclined to disbelieve 

 him, for we know that only elementary species breed 

 true. We explain that the practical breeder bases 

 his selection on fluctuations, and therefore his new 

 race is bound to revert to type. It is obvious now 

 that there is a flaw in this argument. The practical 

 breeder may be basing his selection on fluctuations, but 

 at the same time he may be piling up cumulative fac- 

 tors in the right direction. Thus he might eventually 

 secure a race containing all the cumulative factors. 

 Such a race would be a homozygote and could not help 

 breeding true. Most of the claims of artificially im- 

 proved races that breed true may be false, but it should 

 be remembered that such a thing is possible and may 

 be stumbled upon accidentally, even with unscientific 

 breeding. 



There is another phenomenon which has been 

 much discussed, and which can now be explained in 

 the same way. This is the so-called fixation of hybrids. 

 For years breeders have made promiscuous crosses and 

 then begun artificial selection with the F 2 generation. 

 Eventually they have secured a pure-breeding hybrid. 

 It will be remembered that it was in this way EAST 



