CHAPTER VI 



EVOLUTION OF NEW BACES BY VABIATIONS IN THE 

 POTENCY OF CHAEACTEBS 



IN the last chapter we discussed the color 

 variations of mammals, and we concluded 

 that these result largely from the loss or 

 modification of some half-dozen independent 

 Mendelian unit-characters. As to the material 

 basis of these unit-characters some interesting 

 evidence has recently been collected by Riddle. 

 Melanin pigment has been for some time known 

 to be formed by oxidation. A variety of or- 

 ganic compounds may undergo oxidation into 

 melanin pigments ranging in intensity from 

 light yellow to black; the greater the oxida- 

 tion, the darker the product. But it is not 

 certain, as assumed by Riddle, that the chemi- 

 cal method of oxidation is the same in all cases 

 or that the substance to be oxidized is the same. 

 The results obtained from breeding experiments 



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