slender and heavy; one cannot have both brown eyes and blue. In
some cases, the child resembles one parent and not the other. In other
cases, the child looks somewhat like both parents but not exactly like
either. If one parent is white and the other black, the child is
neither as white as the one parent nor as black as the other.

The parents of a child are themselves different, but there are four
grandparents, and each of them different from the others. There are
eight great grandparents, and all of them different. If we go back only
seven generations, covering a period of perhaps only a hundred and fifty
years, we have one hundred and twenty-eight ancestors. If we go back ten
generations, we have over a thousand ancestors in our line of descent.
Each of these people was, in some measure, different from the others.
Our inheritance comes from all of them and from each of them.

How do all of these diverse characteristics work out in the child? In
the first place, it seems evident that we do not inherit our bodies as
wholes, but in parts or units. We may think of the human race as a whole
being made up of a great number of unit characters. No one person
possesses all of them. Every person is lacking in some of them. His
neighbor may be lacking in quite different ones. Now one parent
transmits to the child a certain combination of unit characters; the
other parent, a different combination. These characteristics may not all
appear in the child, but all are transmitted through it to the next
generation, and they are transmitted purely. By being transmitted
purely, we mean that the characteristic does not seem to lose its
identity and disappear in fusions or mixtures. The essential point in
this doctrine of heredity is known as Mendelism; it is the principle of
inheritance through the pure transmission of unit characters.

An illustration will probably make the Mendelian principle clear. Let us
select our illustration from the plant world. It is found that if white
and yellow corn are crossed, all the corn the first year, resulting from
this crossing, will be yellow. Now, if this hybrid yellow corn is
planted the second year, and freely cross-fertilized, it turns out that
one fourth of it will be white and three fourths yellow. But this yellow
consists of three parts: one part being pure yellow which will breed
true, producing nothing but yellow; the other two parts transmit white
and yellow in equal ratio. That is to say,