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THE AMERICAN NATURALIST [Vol. XLIV 



Why this difference! Is there one law for the Jews, 

 another for the Gentiles ? 



Looking into the matter with care, we find that the 

 results with our own material are, after all, like those 

 of the investigators mentioned if we treat it in the same 

 way. None of these workers first isolated their pure 

 races. If we begin with a mixture we can, in beans, in 

 barley, in Paramecium, in Hydra, by a methodical proc- 

 ess of slow selection make gradual progress in a certain 

 direction. But our selection is only a process of purifica- 

 tion, and when we finally get a pure race, selection is 

 utterly powerless to go farther. We should have been, 

 completely in the dark as to the real effect of selection 

 if we had not carried through rigidly the 1 'pure line" 

 idea. 



Is it possible then that we have in this pure line idea 

 an instrument of the greatest importance for analysis? 

 Is it perhaps the key which every one must have in order 

 to understand the results of selection ? May it be indeed 

 one of those fundamental ideas which, like the idea of 

 mutation, is fitted to clear and crystallize a confused and 

 turbid mixture! Is it possibly of sufficient importance 

 to deserve agitating a little before the American Society 

 of Naturalists? 



Let us put these questions to the practical test; let 

 us apply the idea as an instrument for the dissection of 

 the classic cases which seem to demonstrate the efficacy 

 of selection in producing change of type. 



Johannsen in his recent book has used the pure line 

 concept as an instrument for analysis of the entire field 

 of variation, heredity and evolution, and to him is due 

 the credit of first perceiving the importance of this con- 

 cept, when sharply defined, as such an instrument for 

 research and presentation. The work of Johannsen, I 

 believe, will remain one of the landmarks of progress 

 in this field. But my own analysis has been independent 

 of Johannsen 's and diverse from it, developing inevit- 

 ably from what I have myself seen, so that I may ven- 

 ture still to present some of its results. 



