enture the guess that some of the specific and varietal
differences that are characteristic of wild types and which at the same
time appear to have no survival value, are only by-products of factors
whose most important effect is on another part of the organism where their
influence is of vital importance.

It is well known that systematists make use of characters that are constant
for groups of species, but which do not appear in themselves to have an
adaptive significance. If we may suppose that the constancy of such
characters may be only an index of the presence of a factor whose _chief_
influence is in some other direction or directions, some physiological
influence, for example, we can give at least a reasonable explanation of
the constancy of such characters.

I am inclined to think that an overstatement to the effect that each factor
may affect the entire body, is less likely to do harm than to state that
each factor affects only a particular character. The reckless use of the
phrase "unit character" has done much to mislead the uninitiated as to the
effects that a single change in the germ plasm may produce on the organism.
Fortunately, the expression "unit character" is being less used by those
students of genetics who are more careful in regard to the implications of
their terminology.

There is a class of cases of inheritance, due to the XY chromosomes, that
is called sex linked inheritance. It is shown both by mutant characters and
characters of wild species.

For instance, white eye color in Drosophila shows sex linked inheritance.
If a white eyed male is mated to a wild red eyed female (fig. 35) all the
offspring have red eyes. If these are inbred, there are three red to one
white eyed offspring, but white eyes occur only in the males. The
grandfather has transmitted his peculiarity to half of his grandsons, but
to none of his granddaughters.

[Illustration: FIG. 35. Diagram showing a cross between a white eyed male
and a red eyed female of the fruit fly. Sex linked inheritance.]

The reciprocal cross (fig. 36) is also interesting. If a white eyed female
is bred to a red eyed male, all of the daughters have red eyes and all of
the sons have white eyes. We call this criss-cross inheritance. If these
offspring are inbred, they produce equal numbers of red eyed and white eyed
females and equal numbers of red eyed and white eyed males. The ratio is 1:
1: 1: 1, or ignoring sex, 2 reds to 2 whites, and not