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THE AMEBIC AN NATURALIST [Vol. LIV 



demonstrate that the amount of spotting in the offspring 

 is not correlated with the amount of spotting in the 

 parents. Being due to non-genetic causes, the variations 

 within the type should appear purely at random. An ex T 

 cellent example of this kind of variation has "been com- 

 municated in a paper read before the American Society 

 of Naturalists at their December, 1919, meeting by Dr. 

 Sewall "Wright. The piebald pattern of guinea pigs ap- 

 peared in his experiments to be determined primarily by 

 one recessive Mendelian gene, the expression of which is 

 altered through extremely wide ranges by environment, 

 sox and the uncontrolled vagaries of development. 



The second explanation presumes the occurrence of 

 modifying genes separable in heredity from other genes 

 for spotting, yet only coming to expression in the pres- 

 ence of the main gene or genes for spotting. If this be 

 true, the various grades of spotting within any type such 

 as black-eyed white should be heritable. The amount of 

 spotting in the offspring should be definitely correlated 

 with the amount of spotting in the parents. Whiter 

 black-eyed whites, for example, should have whiter off- 

 spring; and darker black-eyed whites should have darker 

 offspring. Little has found some evidence that this may 

 be true of certain grades of piebald, for he noted that the 

 offspring of hybrids between selfs and piebald tended to 

 cluster about the mode of the particular piebald grand- 

 parents. 



The experimental test of these diverse explanations 

 has in the present experiments consisted of crossing 

 mice of one spotted type with lighter (much spotted) 

 and darker (little spotted) mice of another type. The 

 offspring of the lighter and darker matings have then 

 been compared as to mean and range. Some matings 

 of lighter types inter se and darker types inter se have 

 been made, and while these have not yet yielded enough 

 offspring to have a decisive bearing on the question, 

 they indicate that black-eyed white and piebalds with 

 more white spotting than the mean of their respective 

 types and with amounts of spotting below the mean 



