No. 520] BASIS OF MENDELIAN PHENOMENA 205 



able that the explanation of Mendelian phenomena is to 

 be sought in the nature of the character itself, which 

 conditions and perhaps determines this type of in- 

 heritance. 



Another important point in connection with this hybrid 

 is that the split in the F 2 does not produce a return to 

 the condition of one of the grandparents, as in typical 

 Mendelian behavior, but the difference between the two 

 types of the F 2 —0. rubricalyx and 0. rubriner vis— is a 

 purely quantitative difference, in capacity for pigment 

 production, although morphologically this difference is 

 expressed in certain very definite regions of the plant. 

 There are no intermediates between these two types, 

 each being an independent condition of stability. The 

 difference, therefore, clearly dates back to the germ cells. 



That a quantitative difference between germ cells in 

 their capacity for pigment production can behave in 

 Mendelian fashion, showing the phenomena of dominance 

 and segregation, is of fundamental significance in the 

 interpretation of the nature and material basis of these 

 phenomena. 



Riddle ('09) has brought together in an interesting 

 way the facts of physiology and biochemistry which show 

 that the various pigment colors in mammals and other 

 animals are different stages in the oxidation of a single 

 melanin pigment by the enzyme tyrosinase. In the light 

 of the facts of quantitative Mendelian inheritance which 

 I have presented, it is clear that color inheritance in 

 the mammals can also be most easily interpreted as a 

 case of quantitative inheritance, due to initial quantita- 

 tive differences of some sort in the germ cells themselves. 

 It is not impossible that nearly all Mendelian inheritance 

 may be found, on sufficient analysis of the characters, to 



apparently qualitative differences, when analyzed, are 

 found to be purely quantitative in origin. It should be 

 remembered in the case of melanin pigments, as Riddle 

 also points out, that while some tyrosin oxidations lead 

 to the formation of melanins, others result in the usual 



