100 



THE AMERICAN NATURALIST [Vol. L 



VII 



Turning now to the general problem of selection there 

 are certain fundamental matters which it seems to me are 

 in danger of being lost sight of in the rapid shiftings of 

 view point which are an essential part of any general con- 

 troversial campaign, such as Professor Castle's writings 

 of the last few years would indicate that he engaged in. 

 These are: 



1. The pure-line concept has certainly been one of the 

 most useful working tools in the practical breeding of 

 plants and animals that has ever appeared. Particularly 

 in plant breeding the pioneer work at Svalof, which has 

 been repeated and duplicated on a most extensive scale in 

 plant breeding laboratories all over the world, demon- 

 strates in the most complete manner that, whatever may be 

 happening in the germ-plasm of rats, certainly the germ- 

 plasm of our common cereal crops is in such a state or 

 condition that selection within the pure line is without 

 effect. This is a fact, real and definite. It lies definitely 

 at the basis of very extensive commercial seed breeding 

 operations in various different countries. To any one 

 familiar with the extent and stability of the practical ap- 

 plications of the pure-line concept in cereal breeding op- 

 erations, some of our current discussions of the selection 

 problem seem very academic indeed. Even the justly 

 celebrated magnitude of Castle's rat experiments is 

 scarcely of the same order as the combined and accordant 

 experience of expert cereal breeders throughout the world. 

 Before any one makes up his mind finally about the prob- 

 lem of the efficiency of selection within the pure line it 

 will be well to remember that besides Johannsen's famous, 

 if now in certain quarters somewhat distrusted, beans, 

 there are all the Svalof oats, wheats, etc., to be reckoned 

 with. 



2. No one has ever disputed the power of systematic 

 selection to alter populations, which were not pure-lines. 

 Such alteration may extend the range of variation very 

 greatly beyond what it was in the original population. 



