SPECIFIC DIFFERENCES VS. MUTATIONAL DIFFERENCES.     119

XIV. SPECIFIC DIFFERENCES VS. MUTATIONAL
DIFFERENCES.

An examination of the keys given in this paper will show that the
differences most frequently used are those involving the number and
relative sizes of the bristles, the details of venation, the color of the
wings, of the mesonotum, and to a less extent of other parts of the
body, and the arrangement of the hairs. Other characters in which
marked differences exist in the living animals include eye-color, wing-
shape, abdominal pattern, the she and shape of the eyes, and the
genitalia of both sexes.              - ■

Bridges (1919) has estimated the relative frequency of certain types
of viable mutants in D. melanogaster. He finds that about 25 per cent
of the mutant genes studied have affected the wings (size, shape, or
venation), about 20 per cent the color of the eyes, and about 16 per
cent the general body-color. The two next most frequent types are
undoubtedly those affecting the number, size, or distribution of the
bristles or hairs, and those affecting eye-shape or texture.

There is a large psychological factor in the data relating to the
characters in which species differ, and also in the results obtained by
Bridges for mutant characters. The keys are drawn up to fit pinned
material, so that eye-colors or genitalia, which usually can not be
studied in such material 'are at once eliminated. In studying specific
differences it often becdfnes necessary to examine minute characters,
such as wing-vein indices or the relative sizes of certain bristles, that
are seldom examined in material bred for genetic purposes. The shape
of the carina is an excellent specific character, but is difficult to examine
in unpinned material; no mutations in it have been observed. Simi¬
larly, the immature stages and the food and mating habits, that differ
markedly among the species, have never been really examined for
mutations.

For these reasons it is not practicable to compile statistics showing
the relative frequency of occurrence of different kinds of characters
among the species and among the mutations. We can, however, see
if the mutant characters that have been observed are similar to char¬
acters found in wild species. That many of them are similar will
appear from the following list of special cases. This list is by no means
complete, but includes enough striking cases to illustrate the point
under discussion.

SPECIFIC  CASES OF  PARALLELISM  BETWEEN  MUTANT
CHARACTERS AND CHARACTERS OF WILD SPECIES.

Size: The species of Drosophila differ considerably in size, ranging from
about 1 mm. to about 6 mm. in length. Among the mutant characters of
D. melanogaster are at least two dwarfs and one giant, and in D. simulans