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THE AMERICAN NATURALIST 



[Vol. L 



selection, as an agency in evolution, must then be restored to the im- 

 portant place which it held in Darwin's estimation, an agency capable 

 of producing continuous and progressive racial changes (Castle, 15b, 

 p. 97). 



Castle's experiments have justly become famous. For 

 eight years they have been continuously in progress ; they 

 have involved large amounts of arduous labor; they have 

 been conducted with unflagging zeal and high ideals of 

 scientific attainment. The conclusions drawn from such 

 an important investigation should receive painstaking 

 consideration. 



The writer has been conducting selection experiments 

 which have led him to conclusions different from those 

 reached by Castle. Although these experiments have not 

 involved the expenditure of so much labor and time as did 

 Castle's work, they include three times as many genera- 

 tions and four times as many individuals as are reported 

 by Castle. One investigation was on rats, the other on 

 flies, yet there are so many similarities in the results that 

 the writer was led to make a careful analysis of Castle's 

 papers in an attempt to discover the basis for the conflict- 

 ing conclusions. The final result of this study was to 

 make the writer feel that the following statements in re- 

 gard to the hooded rats are too positive. 



All the evidence we have thus far obtained indicates that outside 

 modifiers will not account for the changes observed in the hooded pat- 

 tern, itself a clear Mendelian unit (Castle, :15b, p. 722). 



. . . there can be no doubt that only a single genetic factor is here 

 involved (Castle, :16, p. 95). 



It is precisely this last named category of cases [a single factorial 

 basis undergoing quantitat ivo variation] which alone can explain our rat 

 results (Castle, :15b, p. 725). 



Energetic attacks have been made on the interpretation 

 Castle has given to his results, and certain unwarranted 

 criticisms have been duly answered. * That the theory of 

 multiple factors may be applied to the results as pub- 

 lished in 1914 in the Carnegie Institution Publication No. 

 195 was indicated therein by Castle, and further empha- 

 sized by Muller ( :14). Most of the criticisms of the ex- 

 periments with the hooded rats have been based on the 



