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 size 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 becomes 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 
