MUTATION 169 



and can not be made to intergrade by crossing. 

 An instance will show how these characters be- 

 have. When a black fowl is crossed with an 

 albino, of the Silky race, the offspring are black 

 with a trace of red in the males. When the hy- 

 brids are mated together they yield albinos, solid 

 blacks, blacks marked with red, and typically 

 colored red-and-black Games. If you keep on 

 crossing together the red-ticked blacks you 

 always get albinos, solid blacks, red-ticked blacks 

 and Games, and nothing else. Such an experi- 

 ence makes clear, better than any argument, the 

 meaning of unit character, discontinuity, and mu- 

 tation. Further analysis of this case shows that 

 the black fowl has a unit character — ^melanic 

 super-pigmentation — ^that has been added to the 

 primitive Game coloration; and the albino lacks 

 a unit character — ^the pigment forming enzyme 

 — foimd in the ancestral plumage. Neither of 

 these unit characters blends in the crossing. If 

 now these unit characters of normal plumage 

 color, excessive melanism, and albinism are to- 

 day non-blending, essentially unalterable char- 

 acters, it is probable that they have always been 

 so and were so in their origin. But we have 

 direct evidence as to this matter. In discussing 

 the case of the black-shouldered peacock, Darwin 

 concludes : " The case is the most remarkable 

 one ever recorded of the abrupt appearance of 

 a new form." If the black peacock arose sud- 

 denly, so probably did the first black Mediterra- 



