erges its size
remains nearly constant, as in many insects. Two races have, however, been
separated by Bridges that are different in size as a result of a genetic
factor. The first of these, called dwarf, is represented by figure 55, (b).

The race is minute, although of course its size is variable, depending on
food and other conditions. The same figure shows the presence of another
factor, "sooty", that makes the fly very dark. Maroon eye color might be
here represented, due to still another factor.

In the third figure (c) the other mutation in size is shown. It is called
"giant". The flies are twice the size of wild flies. An eye color, called
peach, might here be represented. It is an allelomorph of pink.

In the fourth figure (d) the mutant called dichaete is shown. It is
characterized by the absence of two of the bristles on the thorax. Other
bristles may also be absent, but not so constantly as the two just
mentioned. Another effect of the same factor is the spread-out condition of
the wings. The very dark eye color in this figure may be taken to indicate
the presence of another factor, "sepia", which causes the eyes to assume a
brown color that becomes black with age. Most of the other mutations in eye
color that have occurred tend to give a lighter color: this one, which is
also recessive, makes the eye darker.

In the fifth figure (e) the color of the darkest fly is due to a factor
called ebony, which is an allelomorph of sooty.

In the sixth figure (f) the wings are beaded, i.e., the margin is defective
at intervals, giving a beaded-like outline to the wings. This condition is
very variable and much affected by other factors that influence the shape
of the wings. The lighter eye color of the drawing may be taken to
represent pink.

In the seventh figure (g) the wings are curled up over the back. This is a
recessive character.

_Group IV_

Only two mutants have been obtained that do not belong to any of the
preceding groups; these are put together in Group IV. It has been shown
that they are linked to each other and the linkage is so close that it has
thus far been impossible to obtain the dominant recessive. One of these
mutants, called "eyeless" (fig. 56, a, a^1), is variable--the eyes are
often entirely absent or represented by one or more groups of ommatidia.
The outline of the original eye, so to speak, is strongly marked out and
its area might be called a rudimentary organ, if such a statement h