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THE AMERICAN NATURALIST [Vol. XLVIII 



single process, one greater, the other less, than as the 

 presence and absence of one or more unit characters. 



Hagedoorn's Work 



Hagedoorn's work shows the danger of the modern 

 tendency to produce factors upon the slightest provoca- 

 tion. While adding, in experimental work, only a single 

 litter of young bearing on the problem, he gives a symbol 

 for a factor for dominant spotting in mice, and further 

 considers it as due to a factor similar to that producing 

 the dominant " English" spotting of rabbits. He refers 

 to Morgan's work with black-eyed white and self mice 

 as being a study of this dominant factor in mice. Mor- 

 gan himself suggests that if black-eyed white mice repre- 

 sent the extremes of the spotted series the appearance 

 of spotted animals in crosses with selfs is due to a 

 strengthening of the spotting factor or to a change in 

 dominance. This is far different from supposing the 

 addition of an entirely new inhibiting factor comparable 

 to the English pattern in rabbits. Cuenot with mice and 

 Castle (1905) with guinea-pigs have shown that black- 

 eyed whites are the extreme of the recessive spotted 

 series and it is almost certain that Morgan's explanation 

 of the results, as due to a change in dominance, is the 

 correct one. It is, of course, obvious that the presence 

 and absence hypothesis fails to explain any change of 

 dominance of a single character. 



To treat "dominant" spotting in mice as due to the 

 presence of a definite unit-character is exceeding present 

 experimental facts, while to consider it similar in nature 

 to the "English" spotting of rabbits is still less justified. 



One other point in Hagedoorn's work is of such a 

 nature as to require further experimentation before it 

 can be accepted. 



This is the case (on page 126) of "mutual repulsion be- 

 tween two factors." In this case, Hagedoorn mated to- 

 gether agouti animals heterozygous in factor A (for color 

 production) and in factor G (for the agouti pattern). 



