MASKED CHARACTERS 203 
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 masking of characters, or crypto- 
merism, the latter being the term employed by Tscher- 
mak, who was the first to describe these phenomena 
in connection with Mendelian ratios. 
In the simpler cases an invisible or masked factor 
derived from one patent, 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 
to the building up of a type which is a homozygote in 
respect of both the necessary factors. 
This may be made clearer by a definite illustration. 
A pea-plant characterized by the presence of a greyish 
