12            THE  NORTH AMERICAN  SPECIES  OF  DROSOPHILA.

III. GENETICS.

Drosophila melanogaster has been more extensively used in the study
of genetics than any other organism, and the theory of heredity that
is now generally accepted is based chiefly on the results obtained with
this fly.

The first paper on the genetics of Drosophila was published in 1906
(Castle et al.); the first mutation in D. melanogaster was reported in
1910 (Morgan). Since that time about 150 books and papers dealing
with heredity in the genus have been published. About 250 different
mutant types have been discovered and studied, and at least 10,000,000
living individuals have been etherized and examined by more than a
score of investigators. The problems studied include practically every
branch of the subject of genetics. Not only has Drosophila been the
most productive material for research in the subject, but it is now the
standard object for laboratory instruction, and is used as such in
many colleges and universities.

The mutant-types produced by D. melanogaster are of very many
kinds. Eye-colors ranging from pure white to deep sepia are known,
and general body-colors from pale yellow to dark black-and-brown.
The shape of the eye and the character of its surface are both affected
by mutation. The wings are shortened, changed in shape, have parts
lost, or are entirely wanting. The bristles may be deformed, increased
or decreased in number, or made small. The microchsetse may be
disarranged, reduced in number, or may occur in areas normally bare.
Extra veins may occur, or veins normally present may be thickened,
weakened, displaced, reduced, or lost. The legs may be misshapen,
have missing parts, or be increased in number. Giants and dwarfs
are both known. There are types that always die before meta¬
morphosis, and there are types in which the females are always sterile;
others in which the males are always sterile. All these and many
others have been shown to differ from the usual or "wild-type" form
by definite, heritable, and relatively stable units known as genes.

New or "mutant" genes arise only rarely, and their appearance is
not under control. Once arisen, they are perpetuated simply by
breeding from individuals that bear them. It has been found that the
250 or more genes known in D. melanogaster are not inherited entirely
independently, but fall into four groups. The members of any one
group are entirely independent of all members of any other group, but
are more or less closely associated in inheritance with the members
of their own group. These four groups correspond in size and in
many other ways with the four chromosome pairs of this species;
and it seems certain that the genes in any one group are associated
because they lie in the same chromosome pair, and for no other reason.