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Indiana University Studies 



results hitherto reached in a study of uniparental inheritance, 

 a test that would meet the criticisms hitherto made, by em- 

 ploying an organism not open to these criticisms. In this 

 favorable organism, as we have seen, the results are opposite 

 to those commonly reached; gradual hereditary differentia- 

 tion occurs. The direct, simple, and natural conclusion is 

 that the experiments have supplied precisely the test they 

 were designed to supply and have given clear results. By 

 working with clearly marked characters, and by excluding 

 growth stages and environmental modifications; by basing 

 selection entirely on congenital characters, and continuing it 

 through a great number of generations, we have found that 

 in these organisms the genotype is not constant, but changes 

 by slow gradations, such as would not be revealed by imper- 

 fect selection for a few generations." 



The facts in these cases seem clear, but the interpretation 

 of Jennings' results have been questioned by Morgan ('16) 

 and Pearl ('17). Morgan says: "If through sexual union 

 the germ plasm of these wild types has in times past been 

 recombined, then selection would be expected to separate cer- 

 tain types again, if, at division, irregular sampling of the 

 germ plasm takes place. Until these points are settled the 

 bearing of the general problem on heredity is uncertain." 

 Pearl suggests that when Jennings changed his method of 

 selection at the end of the sixth generation in family No. 326 

 and based it to a considerable extent on past performance, 

 he changed his basis of selection from the soma to the germ. 

 In the first six generations, selection produced no results, 

 but after the basis of selection was changed, selection was 

 effective. 



And so the question still remains an open one with the 

 balance of evidence, I think, in favor of the multiple factor 

 hypothesis. 



Perhaps some of the differences of opinion are due to 

 misunderstandings as to terminology and meaning, and if 

 some of these differences were cleared away we might be a 

 little nearer to a common viewpoint. Jennings ('17b) has 

 attempted to clarify the atmosphere and I think has done it 

 remarkably well, except that he leaves one with the general 

 impression that there still exist two enemy camps. Castle and 

 himself on one side and the mutationists on the other. 



