VARIATION AND HEREDITY 233 



have thrown on the subject, it is reasonable to believe 

 that reversions are merely the reappearance of unit 

 characters which have been latent, generation after 

 generation, because the proper combination of causal 

 factors or the removal of an inhibiting factor has 

 not occurred in mating. The factor has been " passed 

 on," however, from parent to offspring under the 

 surface, so to speak, until the disturbing hand of the 

 experimenter has set new. combinations or removed 

 the inhibitions. 



Sex-limited Inheritance. - - Experiments have 

 shown that, in many cases, the correlations of somatic 

 characters are only explicable on the assumption 

 that the factors for these characters are coupled 

 or linked together, and that one is not manifest 

 without the other. A striking example of such a 

 correlation is found in the fact that certain charac- 

 ters are only manifest in one sex although trans- 

 mitted through both sexes. This involves the further 

 assumption that sex itself is a Mendelian allelomorph, 

 an hypothesis that had been advanced earlier on 

 cytological grounds. Thus, color-blindness in human 

 beings is a condition much more prevalent in men 

 than in women, and it has been found that whereas 

 a woman, a daughter of a color-blind man, may 

 not reveal this character herself, she will transmit 

 it to her sons, who will be color-blind. On the other 

 hand, her daughters, though capable of passing on the 

 trait, will, themselves, not be affected. 



A more striking example is found in the little 



