MUTATION. 143 



treraes, we would not know whether this grey should be inter- 

 preted as 50 % black, due to the presence of only one dose of a 

 black-making gene, or as 50 % white, due to one dose of a 

 white-making gene, which the white parent had more than the 

 black one. And a case in which the hybrid between a black and 

 a white were 70 % black, might also be interpreted as 30 % 

 white, caused by insufficient action of one dose of a white 

 making gene. There would be no way to decide which of the 

 crossed individuals, the black or the white, had a gene more 

 than the other. Now the fact is, that, whereas we know a few 

 instances in which a heterozygote is just noticeably disting- 

 uishable from a homozygote, there are no cases in which 

 dominance in a heterozygote is about 80 % or 60 % only, and 

 in which we can be sure that there are not more than one'gene 

 responsible for the difference. For here lies the difficulty. If 

 we observe a mulatto who is 60 % as black as his negro father, 

 we do not know whether this father differed from the white 

 mother only in one black-making gene. We know, to confine 

 ourselves to colour, that there are genes which tend to make 

 pigmentation deeper, and that there are others which make it 

 lighter, so that if two animals or plants are crossed, of which 

 one has a darkening gene and the other a lightening one, and 

 each lacks the other, the colour of the hybrid may be inter- 

 mediate as a result of the fact that the two genes counteract 

 each other's effect upon the development. A few instances: 

 Black mice have a gene more than certain yellows, but they 

 lack a certain other gene which agoutis have. The same relation 

 is found in rabbits. Yellow agouti mice and rabbits have 

 this last gene, but they lack the one which blacks have more 

 than yellows. If we now mate a black to a yellow agouti, the 

 hybrids will be agouti, which is as exactly intermediate be- 

 tween the colours of the parents as one could only wish. Bate- 

 son found that the Leghorns have a gene, which tends to re- 

 duce the pigmentation of the skin. The black-skinned Silkies 

 lack this gene, but possess another, which makes possible an 

 intense blue-black pigmentation of the skin. Hybrids between 



