By heredity we mean the likeness between parent and offspring. This
likeness is a matter of form and structure as well as likeness of action
or response. Animals and plants are like the parents in form and
structure, and to a certain extent their responses are alike when the
individuals are placed in the same situation. A robin is like the parent
robins in size, shape, and color. It also hops like the parent birds,
sings as they do, feeds as they do, builds a similar nest, etc. But the
likeness in action is dependent upon likeness in structure. The young
robin acts as does the old robin, because the nervous mechanism is the
same, and therefore a similar stimulus brings about a similar response.

Most of the scientific work in heredity has been done in the study of
the transmission of physical characteristics. The main facts of heredity
are evident to everybody, but not many people realize how far-reaching
is the principle of resemblance between parent and offspring. From
horses we raise horses. From cows we raise cows. The children of human
beings are human. Not only is this true, but the offspring of horses are
of the same stock as the parents. Not only are the colts of the same
stock as the parents, but they resemble the parents in small details.
This is also true of human beings. We expect a child to be not only of
the same race as the parents, but to have family resemblances to the
parents--the same color of hair, the same shape of head, the same kind
of nose, the same color of eyes, and to have such resemblances as moles
in the same places on the skin, etc. A very little investigation reveals
likenesses between parent and offspring which we may not have expected
before.

However, if we start out to hunt for facts of heredity, we shall perhaps
be as much impressed by differences between parent and child as we shall