THE 



AMERICAN NATURALIST 



Vol. LIV. March-April, 1920 No. 631 



ARE THE FACTORS OF HEREDITY ARRANGED 

 IN A LINE? 



DR. H. J. MULLER 

 Columbia Uxiversity 



In the February (1919) number of the Proceedings of 

 the > Xat ional Academy a}' Science*, Professor Castle states 

 that he has "shown that the arrangement of the genes 

 in the sex-chromosome of DrosopliUa ampclophila is 

 probably not linear, and a method has been developed for 

 constructing a model of the experimentally determined 

 linkage relationships." 1 This declaration is so widely at 

 variance with the conclusions jointly agreed upon by all 

 Drosophila workers, that the arguments or assumptions 

 which it involves would seem to call for careful examina- 

 tion. It may be stated at the outset that the principle 

 upon which Professor Castle constructs his models ap- 

 pears exceedingly direct and simple— it is merely to 

 make a figure such that the distances between all the 

 points represented on it are exactly proportional to the 

 frequencies of separation actually found between the 

 respective factors in the most reliable experiments. If 

 this is done, Castle contends, the models will be three- 

 dimensional instead of linear in shape. 



L The first argument which Castle gives against the 

 view that the groups of genes (which he admits, at least 



1 Sturtevant, Bridges and Morgan also have published a defense of the 



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