126 HEREDITY [ch. 



there are two kinds of eggs, male-bearing and female- 

 bearing. In other cases, however, exactly similar 

 evidence shows that the germ-cells of the male 

 (spermatozoa) are of two kinds. The example of this 

 which has been most fully worked out is the Fly 

 Drosophila, in which a series of remarkable dis- 

 coveries have been made in America by Morgan. He 

 has found several sex-limited characters, perhaps the 

 simplest of which is eye-colour. The normal flies 

 have red eyes ; a white-eyed male appeared and when 

 mated with a red-eyed female gave only red-eyed 

 oflfepring, both males and females. When, however, 

 these were mated together, all the female oflPspring 

 were red-eyed, but about half the males had white eyes. 

 A white-eyed male mated with a heterozygous red- 

 eyed female gave red and white eyes in both sexes, 

 but any red-eyed male mated with a white- eyed 

 female gave all the males white, all the females red- 

 eyed. This case then is in every respect precisely 

 the converse of what is found in the Currant Moth ; 

 the males must be heterozygous for a sex-determiner, 

 and the red factor in the male is transmitted only to 

 the female offspring. 



Apparently • similar conditions are found in Man 

 in certain diseases of the eye, e.g. colour-blindness 

 and certain forms of night-blindness, and in the disease 

 known as Haemophilia. Here the affected male 

 appears to transmit the factor for the disease only to 



