The collection of
these trait-offspring makes up the child.

It has been observed that traits differ with respect to their action in
mating. Given a specific type of trait-mating, say of a trait with like
trait, or trait with the lack of that trait, some types always reappear
in the next generation or else are lost entirely from the family line
unless reinfused, whereas other types of traits may not reappear in the
next generation, but still appear in a generation further removed.
Another type of trait is transmissible only by one sex of a family line,
and can not be transmitted by the other sex.

From these facts, it is readily understandable how important becomes the
consideration of the marriage of relatives, such as cousins,[63] who
are, of course, individuals of the same family line, whose mating
brings together like groups of traits, thus strengthening the existence
of these traits, whether desirable or undesirable. Cousin marriages,
when the family possess traits of mental ability, may result in children
who are geniuses; but cousin marriages, when the family line possesses
traits of mental inability, may result disastrously with respect to
offspring. Family lines possessing traits of mental weakness should most
assuredly join only to family lines possessing traits of strength in
those regards.

In calculating the inheritability of traits, it is also necessary to
consider that certain physical, mental and moral traits flower at the
arrival of certain ages only. It is necessary to look along the whole
line of a life, as traits may exist at one age and not at another. A
boy's beard does not appear until puberty. Likewise, other physical and
mental and moral traits sometimes do not manifest themselves until
specific ages, according to the type of the family breed. Because a
parent dies before the development of the trait does not preclude its
transmissibility to his offspring. Huntington's chorea, an extremely
undesirable trait, does not develop until middle life, but is
transmissible to offspring even though the father dies from some other
cause before the period when the disease in his own person would be
expected to appear.

[Sidenote: Results of Specific Matings]

[Sidenote: Andalusian Fowl]

We can best understand the laws governing the inheritance of traits by
taking a few concrete cases. The first case is that of an Andalusian
fowl. We shall consider the two species, pure bred black and pure bred
whit