the
thorax with its appendages has changed into a segment like the second (fig.
29). It is smaller than the normal mesothorax and its wings are imperfectly
developed, but the bristles on the upper surface may have the typical
arrangement of the normal mesothorax. The mutant shows how great a change
may result from a single factor difference.

A factor that causes duplication in the legs has also been found. Here the
interesting fact was discovered (Hoge) that duplication takes place only in
the cold. At ordinary temperatures the legs are normal.

[Illustration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
normal eye.]

In contrast to the last case, where a character is doubled, is the next one
in which the eyes are lost (fig. 30). This change also took place at a
single step. All the flies of this stock however, cannot be said to be
eyeless, since many of them show pieces of the eye--indeed the variation is
so wide that the eye may even appear like a normal eye unless carefully
examined. Formerly we were taught that eyeless animals arose in caves. This
case shows that they may also arise suddenly in glass milk bottles, by a
change in a single factor.

I may recall in this connection that wingless flies (fig. 5 f) also arose
in our cultures by a single mutation. We used to be told that wingless
insects occurred on desert islands because those insects that had the best
developed wings had been blown out to sea. Whether this is true or not, I
will not pretend to say, but at any rate wingless insects may also arise,
not through a slow process of elimination, but at a single step.

The preceding examples have all related to recessive characters. The next
one is dominant.

[Illustration: FIG. 31. Mutant race of fruit fly called bar to the right
(normal to the left). The eye is a narrow vertical bar, the outline of the
original eye is indicated.]

A single male appeared with a narrow vertical red bar (fig. 31) instead of
the broad red oval eye. Bred to wild females the new character was found to
dominate, at least to the extent that the eyes of all its offspring were
narrower than the normal eye, although not so narrow as the eye of the pure
stock. Around the bar there is a wide border that corresponds to the region
occupied by the rest of the eye of the wild fly. It lacks however the
elements of the eye. It is therefore to be looked upon as a rudimentary
organ, which is, so to speak, a by-product of the do