COLOR. 73 



but above the black threshold on the chest. Increase in production 

 of II produces the silver phase, nearly self sepia in color, including 

 the chest. The colors of the varying hare seem to be due to variations 

 in production of I determined by environmental causes. The white 

 winter pelage gives way in blotches to a white-ticked sepia ; this gives 

 way to yellow-ticked sepia as the intensity of production of the basic 

 enzyme rises above the yellow threshold and in some varieties the full 

 summer pelage is almost self red. 



Many other cases could be given in which two color phases of an 

 individual animal or the color patterns of closely allied varieties seem 

 to differ in many respects and yet can be explained on the basis outlined 

 as due to a single physiological change. 



In the case of very complex color patterns, it is necessary to suppose 

 that the power of producing the hypothetical enzymes I, II, or III 

 may be distributed in quite complex patterns. But the hypothesis 

 often gives a simple explanation for certain peculiarities in a pattern. 

 In the tiger, the stripes on the back are quite intense yellow and black. 

 The yellow stripes grow paler down the sides, becoming white on the 

 lower sides and belly. The black stripes likewise grow lighter down 

 the sides but at the point at which the yellow becomes white, the black 

 stripes suddenly grow more intense, at least in some individuals, to 

 become paler again on the belly. Again, on the legs, which are white 

 on the inside, yellow on the outside, black stripes are visible on the 

 white part, but either disappear completely or leave merely a streak of 

 sooty red on the yellow part. All of this becomes intelligible if we 

 assume that the basic enzyme (I) is produced at decreasing rates from 

 back to belly and from outside to inside of leg, while the black-produc- 

 ing supplement (II) is distributed in vertical stripes (horizontal on the 

 legs). Two parallel stripes give a remarkable reproduction of the 

 variation in black and yellow in the albino series in guinea-pigs. We 

 have the same change from black and intense yellow to sepia and 

 cream, then to darker sepia and white, and finally light sepia and white, 

 illustrating the different thresholds for the appearance of black and 

 yellow and the reduction in intensity of black above the yellow threshold 

 due to the entrance of competition at this point. 



