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THE AMERICAN NATURALIST [Vol. XLIV 



peas melanin production seems to occur only in the later stages 

 of development. 



From a few facts like those above stated Riddle makes the 

 sweeping conclusion that "in an animal that produces melanin 

 color there exists all the machinery necessary to produce a 

 series or scale of these colors. What is actually produced is, 

 in several demonstrated instances, dependent on the physiolog- 

 ical state of the organism. ' ' It does not necessarily follow that 

 this is true in all organisms, or even in all animals, though it 

 must be admitted that such may be the case. There is a good 

 deal of evidence that in some organisms the oxidation processes 

 do not lead through a series of colors, such as Fornier found in 

 tadpoles, but that when color begins to appear at all it is of a 

 definite character and that we have variation only in the amount 

 of that color present. It seems quite probable that in some 

 organisms the mechanism present is limited to the production 

 of a single color character, yet this is a matter for further in- 

 vestigation. 



Even if there is only one process of oxidation and the series 

 of reactions results in a graduated series of color pigments this 

 does not necessarily imply that a tendency to a certain color is 

 not purely hereditary and that it could not undergo the phe- 

 nomena of segregation. The normal stage of oxidation reached 

 in an organism may be an intermediate one, as in red cattle. 

 If a single cell organ is responsible for the oxidation process 

 stopping at this stage, and if that cell organ behaves as chromo- 

 somes appear to do at least in some organisms, then we would 

 necessarily have the phenomenon of Mendelian segregation. 



Riddle seems to labor under the impression that in order to 

 explain the so-called Mendelian factors it is necessary to assume 

 an indefinite number of specific enzymes or chromogens. I 

 think I have already said enough to point out that this is not the 

 fact. There is much evidence, however, that in the color of 

 animals we do have at least three specific enzymes or chrom- 

 ogens, for we find three kinds of color material deposited in the 

 same parts of the organism. Furthermore, the tendency to 

 produce each of these three types of pigments has been demon- 

 strated to be separately heritable. 



Riddle points out the very interesting fact that Miss Durham 2 



2 Proceedings Eoyal Society, Vol. 74. 



