LATENT CHARACTERS 189 



because in true cases of reversion a certain pro- 

 portion of the reversionary individuals of F 2 are 

 found to breed true, which a simple heterozygote will 

 never do. 



It has been found that the essential part of this 

 phenomenon of reversion on crossing consists in the 

 existence in the parents of certain hereditary factors 

 allelomorphs, in fact which, although by themselves 

 invisible, yet, when combined in cross-breeding with 

 certain other allelomorphs, belonging to independent 

 pairs, lead to the appearance of new visible characters. 



The term reversion cannot properly be applied to 

 these phenomena as a class, because, in the first place, 

 characters may arise in this way which cannot be 

 regarded as ancestral, and, secondly, because reversions 

 may take place in other ways ; for example, the 

 reappearance of a simple recessive character would 

 legitimately be ranked among reversions. The best 

 general name for the class of phenomena we are about 

 to describe is perhaps latency of characters, or crypto- 

 merism, the latter being the term employed by 

 Tschermak, who was the first to describe these phe- 

 nomena in connection with Mendelian ratios. 



In the simpler cases an invisible or latent factor 

 derived from one parent, on becoming associated with 

 a different factor born by the other parent, and already 

 visibly represented among the external features of this 

 second parent, makes itself apparent among the visible 

 characteristics of the heterozygote. In such a case 

 the characteristic appearance exhibited by the hetero- 

 zygote may subsequently become permanent, owing 



