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.
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.
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.