MENDELIAN CHARACTERS 213 
a complete account of the phenomenon of reversion 
on crossing, an account which has already been demon- 
strated to hold good in other instances besides that 
of the sweet-pea. The facts are expressed in the 
following manner by their discoverers : ‘ “ Reversion ” 
is thus seen to be a simple and orderly phenomenon 
due to the meeting of factors belonging to distinct 
though complementary allelomorphic pairs, which at 
some moment in the phylogeny of the varieties have 
each lost their complement.’* 
Dr. G. H. Schull has classified the different known 
types of latency in the following manner : 
xr. Latency due to separation. In this form of 
latency an allelomorph, when acting alone, has no 
external manifestation, and is only rendered visible 
by combination with another allelomorph belonging 
to a distinct pair. In this way such ratios as 9: 3: 4 
or 9 : 7 may arise. We have already observed an 
example of this phenomenon in the case of the two 
factors required to produce colour in the sweet-pea. 
2. Rare cases have been described of latency due 
to combination. This, instead of being due to separa- 
tion, is the result of a union in the same zygote of two 
dominant allelomorphs, either of which alone will 
produce a manifest character. When both allelo- 
morphs are present the character fails to appear. 
3. Latency due to hypostasis, or the masking of: 
one dominant character by another character also 
dominant and epistatic to the masked character, has 
already been described in the case of the coat colours 
* Proceedings of the Royal Society, B. vol. 77, p. 238. 
