HEREDITY OF SEX 277 



second of the three species, and to its final disappear- 

 ance in the first. 



Thus, by dint of a good deal of speculation, Wilson 

 and others have arrived at a possible Mendelian descrip- 

 tion of the phenomenon of sex in a species in which 

 the chromosomes of male and female are alike ; and it 

 is a description which has its basis in actual phenomena 

 observed in a number of related animals. 



[In the cases considered above, the female has two 

 similar idiochromosomes and the male either only one, 

 or two which are unlike one another. The female thus 

 produces eggs which are all alike, while the male pro- 

 duces two kinds of spermatozoa, one bearing an idio- 

 chromosome like those of the female, the other either 

 no such chromosome or a dissimilar one. Now, in 

 animals belonging to groups in which phenomena of 

 this sort have been observed, it has also been observed 

 that there are certain characters which are transmitted 

 by the male only to his daughters, while the female 

 transmits them impartially to her offspring of both 

 sexes. The best -known example of this sort is Droso- 

 phila (see p. 270), in which all the characters of group 

 A are transmitted by the male only to his daughters 

 (sex-limited transmission by the male). In Man the 

 same sort of thing occurs in several abnormalities, 

 notably in colour-blindness. Colour-blindness is due 

 to the loss of a factor for normal vision, and this factor 

 for normality is transmitted by a man only to his 

 daughters, but by a woman to her children of both 

 sexes. Hence a colour-blind man married to a norma] 



