EVOLUTION BY LOSS 



by modification without loss of these same 

 factors. 



Yellow varieties owe their origin to a re- 

 duction in the amount of black or brown pig- 

 ment in the fur, and to a corresponding in- 

 crease in the amount of yellow. In some 

 yellow animals, such as the sooty yellow rab- 

 bit, black and brown pigments are not wholly 

 lacking in the fur, but are only greatly re- 

 duced in amount. They always persist in 

 the eye. In other yellow animals, mice for 

 example, the black or brown pigments are 

 wholly absent from the fur, and they may also 

 be greatly reduced in amount in the eye, as 

 in the variety known as pink-eyed yellow, but 

 in no yellow animal, so far as I am aware, is 

 the production of black and of brown pigments 

 wholly suppressed. 



In any mammal which possesses yellow varie- 

 ties we can produce by suitable crosses as many 

 different varieties of yellows as there are of 

 gray, black, cinnamon, and brown varieties 

 combined. For example, in mice, yellow indi- 

 viduals of which, as was shown in the last 

 chapter, are invariably heterozygous and pro- 



75 



