SECT. 3] 



AND ORGANISATION 



611 



had found that the red colour of the eyes in this organism darkened 

 with development almost to black, and, by working with an arbitrary 

 scale of colours, Huxley & Ford were enabled to make a quantitative 

 examination of the rates of action of the various genes which in- 

 fluence eye-colour. Fig. 104, taken from their paper, demonstrates 

 diagrammatically the relationships found. The blackening appears 

 to be due to the deposition of melanin; in some cases this occurs 

 very rapidly, in others more slowly. Thus the steepest curve in the 

 diagram represents the dominant black-eye type which is black at 

 hatching. Embryos of this type, however, which have not completed 

 half their incubation, have no colour in their eyes, but soon they 



Dominant Black Eije. 

 Below thj3 all are 

 Recessive Red 



Moat Rapid Djrieninq 



Me^n Rapid Daitieninq 



Mean Slow Daritenm^ 



Absence of DdHteniiu) 



Fig. 104. 



become pale pink and later scarlet. Just before the end of embryonic 

 life, the eyes darken, until at about the time of extrusion from the 

 brood-pouch they are quite black. It has taken 10 days from fertilisa- 

 tion at 20° to bring this about. Other genetic types, however, hatch 

 with red eyes, and only much later in life approach to blackness, 

 as the diagram illustrates for various varieties. Thus, a definite 

 relation was found to exist between Mendelian genes and rate of a 

 chemical process. 



Morgan, in 1923, discussed such questions as these. "It is to be 

 hoped", he said, "that in time the combined attack on the problem 



