302 ' CONCLUDING CHAPTER 
the heterozygote AB .ab produces in equal numbers 
the germ-cells AB, Ab, aB,and ab. Among the combina- 
tions of these germ-cells which are represented by the 
various offspring of the heterozygote there must appear 
Ab.Ab and aB .aB—novel types which are pure in 
constitution, and which may form the starting-points 
for new strains or races. 
Upon this fact depends the enormous importance of 
Mendel’s law in the breeding of new and useful types 
of animals and plants. When it is remembered that in 
wheat, for example, resistance and non-resistance to 
the attacks of disease, earliness and lateness of 
ripening, good and bad milling quality, are all pairs of 
Mendelian allelomorphs, and that it is now possible to 
take a different example of these qualities from each 
of three different strains, and to combine them together 
in a single new variety with perfect certainty and in 
four generations, it does not require much imagination 
to foresee that every department of the animal and 
plant breeding industries must sooner or later benefit 
enormously from Mendel’s discovery. 
So far we have only been dealing with the very simplest 
of Mendelian phenomena, leading to the arithmetical 
addition and subtraction of definite visible characters. 
Other kinds of allelomorphs also exist which undergo 
a similar process of segregation during gamete forma- 
tion, following Mendel’s law in a perfect manner; but 
which may remain entirely invisible and unsuspected 
so long as certain other allelomorphs, belonging to 
quite distinct pairs, are excluded from the zygotes in 
which these invisible factors are concealed. When 
