TRE 



ALIST [Vol. LIII 



aeters varying .quantitatively or qualitatively. I accept their 

 interpretations as correct in the light of our present knowledge. 



I should feel like apologizing for my own obtuseness in not 

 reaching a similar conclusion sooner, did I not recall with satis- 

 faction how much clearer the role of selection now stands re- 

 vealed than it did when these experiments were begun, and to 

 the clearing up of the situation I shall at least hope that this rat 

 work has contributed something, if only by provoking inves- 

 tigation. 



The "Mutation Theory" of DeVries gave us a picture of se- 

 lection as an agency temporarily etfective in producing racial 

 changes, but with those changes gradually vanishing as soon 

 as the selection ceased. Johannsen denied within "pure lines" 

 even temporary effectiveness of selection, A strictly logical use 

 of Johannsen 's conclusions would have limited their application 

 to such organisms as he studied, self-fertilizing ones completely 

 homozygous for all genetic factors and subject apparently to 

 no new changes in such factors. But the doctrine was straight- 

 way extended in the views of most geneticists to selection of 

 every sort and he was treated as a traitor to Mendelism who saw 

 any utility in selection or advocated its use as a means of im- 

 proving the inherited characters of animals or plants. 



The situation is wholly different to-day. Through the inves- 

 tigations of Jennings and his pupils on protozoa, of Stout on 

 Coleus, and of Shammel on citrous fruits, the fact is clear that 

 even within clones genetic changes may and do frequently occur 

 and that systematic selection will serve to isolate these and thus 

 lead to racial improvement. Those who have tried systematic 

 selection in the ease of cross fertilizing organisms have in some 

 cases noted the occurrence of ' ' mutations ' ' with such frequency 

 as to make progressive change under selection easily obtainable. 

 Emerson and Hayes, in the ease of certain pericarp color pat- 

 terns of maize, find "mutations" so common that a wide range 

 of variability results and selection is able to isolate, from such 

 material, types "relatively stable," but very diverse in appear- 

 ance. Modifying factors are not involved in Emerson's expla- 

 nation of his results, but rather such instability of a single gene 



but utlu'i-wisr similiir- di;! ii-rs. l.lit iiKi.lifyii!- f;i<-t(»rs rather than 

 repeated mutation seeins to lie tiu- t"xi)laiiati()ii rtMiuired in view 

 of the results of crosses reported in this paper. 



