170 ANIMAL LIFE AND HUMAN PKOGKESS 



are silvers. But from the reciprocal mating of gold cock and 

 silver hen the male offspring are all silver and the females 

 all gold. 



Extensive analysis of this and of other cases of similar 

 nature in poultry has led to the conclusion that the female 

 is the heterozygous sex. No silver hen, however bred, has 

 yet been found to be homozygous for the silver factor. 

 Nor, when mated with a gold cock, has such a hen ever been 

 known to give a silver daughter. We are led to suppose, 

 therefore, that she produces two kinds of eggs, viz. those 



. _ X TAKENTS $ 



Ken cock 



i x S 



tan cock 



--- F, 



S 



Cock<5 



hens 



FIG. 7. Scheme illustrating the different results obtained in the 

 reciprocal crosses between gold and silver fowls. For further explanation 

 see text. 



which carry the silver factor but not the factor for femaleness, 

 and those which contain the female factor but not the factor 

 for silver. The fact that she can be proved to be always 

 heterozygous for silver, which we know to be dominant to gold, 

 and the further fact that she can transmit this factor only to 

 her sons, force us to conclude that she is also heterozygous for 

 the factor which decides whether a chicken is to be a hen. 

 Experimental work has also shown that other birds, such as 

 pigeons and canaries, resemble poultry in this respect, as also 

 does the common currant moth. 



Sex is apparently such a deeply seated feature that we 



