Many mutants are, in fact, lethal to their possessors. Mutants which equal the normal fly in vigor are a minority, and mutants that would make a major improvement of the normal organization in the normal environments are unknown. Nilsson, p. 1186. In the best-known organisms, like Drosophila, innumerable mutants are known. Goldschmidt, p. 94.
Fruit flies refuse to become anything but fruit flies under any circumstances yet devised.
Hitching, p. 61. Grass , p. 130. This site best viewed with Netscape 6 or Internet Explorer 4.0 or greater.