THE SCIENCE OF GENETICS 299 
naturally to group themselves is afforded by the law 
of inheritance discovered by the Abbé Mendel about 
the year 1865. This discovery has rendered possible 
that rapid advance of the science of genetics, or the 
study of the hereditary phenomena of organisms, 
which has taken place during the first few years of the 
twentieth century. It is the writer’s avowed opinion 
that Mendel’s brief paper is the most important con- 
tribution of its size which has ever been made to bio- 
logical science. Little apology is therefore needed for 
formulating once again the law based by Professor 
Correns upon the conclusions which this paper contains. 
Mendel’s law relates to the inheritance of certain 
definite characters, which have since been called allelo- 
morphs. It is a distinctive feature of allelomorphic 
characters that they are found to group themselves 
naturally into pairs of more or less antagonistic 
qualities. In many cases the pair is represented by 
the presence and absence respectively of a certain 
definite feature. The two allelomorphs of a pair may 
be conveniently written as A and a. 
We have seen that the cells of zygotic organisms— 
organisms, that is to say, which have arisen by the 
process of sexual reproduction—contain a double 
complement of hereditary qualities. Such cells may 
contain A and A,aanda,orAanda. The forms AA 
and aa are described as homozygotes, the form Aa as 
a heterozygote. In the simpler cases we are enabled 
to study the behaviour of such a single pair of allelo- 
morphs by itself, without reference to any other features 
which the animals or plants under consideration may 
