MENDELISM 195 
of P. sinensis, P. pyramidalis, and P. stellata in the 
ratio of 1: 2:1. 
Cases like the above illustrate the essential part of 
Mendel’s law even better than those in which domi- 
nance is present, the characteristic proportion of one 
of each homozygote type to two of the heterozygote 
being at once recognisable in such a case without the 
necessity for further breeding ; whereas, in cases where 
there is dominance, further study is necessary in order 
to distinguish, among the individuals of dominant 
appearance, those which are pure dominant and those 
which are heterozygous in constitution. 
In concluding our account of the simpler forms of 
Mendelian phenomena we may consider one further 
point with regard to the nature of the two allelomorphs 
making up any particular pair. In what is probably 
a majority of the cases hitherto examined the dominant 
and recessive allelomorph seem to represent respec- 
tively the presence and absence of something. Thus 
the dominance of colour to absence of colour, or white- 
ness, is a very frequent phenomenon. And in some of 
the more complex cases to be described in the next 
chapter we shall find the presence and absence of a 
particular factor very often behaving as a pair of Men- 
delian allelomorphs. The question arises as to how 
far this conception should be extended. It seems, for 
instance, somewhat far-fetched to speak of dwarfness 
as being simply determined by the absence of the factor 
for tallness, though it is not impossible that this may 
be the correct way of looking at the facts. Be this as 
it may, it is to be remembered that a Mendelian pair 
I3—2 
