SHORTER ARTICLES AND DISCUSSION 



PIEBALD RATS AND MULTIPLE FACTORS 



In the Naturalist for December, 1916, MacDowell has pub- 

 lished an extended criticism of experiments in the modification 

 of the hooded pattern of piebald rats by selection, in which my 

 colleagues and I have been engaged for some years. This is not 

 the first time that readers of the Naturalist have had their at- 

 tention called to these experiments by similar adverse criticism 

 and they are possibly quite weary of the subject. In so far as 

 MacDowell merely offers in new form arguments which have 

 already been presented by MuUer and Pearl and answered by 

 me, I shall make no reply. But as regards two points which 

 may fairly be considered critical, one of which actually is so 

 designated by MacDowell, I desire to present some evidence 

 which I regard as conclusive but which MacDowell has not dis- 

 cussed, evidence possibly not accessible to many readers of this 

 journal. MacDowell 's criticism is based on the data presented 

 in Publication 195 of the Carnegie Institution (Castle and Phil- 

 lips, 1914), and in a brief paper in The Scientific Monthly 

 (1916). Many additional data are given in Publication 241 of 

 the Carnegie Institution, but this is not considered by Mac- 

 Dowell, although it was issued in September, 1916, as Paper No. 

 26 of the station with which he is connected, nearly two months 

 in advance of his own paper. Had he considered carefully the 

 evidence contained in this later publication, I am sure that he 

 would have modified his criticism materially. 



In 1914 Phillips and I offered two alternative explanations of 

 the progressive changes observed under selection in the hooded 

 pattern. These were (a) variability of the unit-character (''fac- 

 tor") hooded, and (6) multiple modifying factors affecting 

 the hooded character. We found it difficult to decide between 

 these two interpretations on the basis of evidence then avail- 

 able. For this hesitancy we were promptly taken to task by 

 Muller, who championed the multiple factor interpretation now 

 adopted also by MacDowell. MacDowell elaborates in much de- 

 tail a dozen points which show compatibility between our ob- 

 servations and a multiple factor hypothesis, but without consid- 

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