SPECIFIC DIFFERENCES VS. MUTATIONAL DIFFERENCES. 121 
In some of the Phoride (e. g., Pulicophora, see Williston, 1908, Man. N. 
Amer. Dipt., p. 236) and in the mutant race ‘‘apterous”’ the wings are 
entirely missing. 
Many of the mutant races differ from the wild type in venation. Some 
of these are similar to forms found in nature. 
In “plexus” and in “‘blistered” the fourth vein bends forward sharply 
at its apex, as in the trypetine Anastrepha. 
In “notch” the third vein is thicker than the second and fourth, as in 
Chlorops and related genera (Chloropine). 
In “short,’’ a mutant race of D. obscura, the fifth vein does not extend 
beyond the posterior cross-vein. In “‘hairless” and “abrupt” in D. melano- 
gaster it extends a short way beyond the cross-vein, but not to the wing- 
margin. These conditions can be matched in different species of the 
borborine genus Borborus. 
In “fused”? D. melanogaster the third and fourth veins are more or less 
convergent at their apices, suggesting Phortica and several other muscid 
types. 
In cross-veinless a mutant race of D. virilis, the posterior cross-vein is 
missing, as in the asteine Asteva and the agromyzine Phytomyza. 
In ‘‘short”’ D. tmmigrans the costa is not broken at the apex of the first 
vein. This character occurs in several acalypterate subfamilies. 
Legs: In the mutant race ‘“‘bent,”’ of D. melanogaster, the basal tarsal 
joints are often short and thick, resembling those of the Borborine. 
In “bithorax”’ D. melanogaster the hind leg often bears an apical bristle 
like that of the middle leg. This bristle, however, is usually strongly 
curved and greatly resembles the ‘“‘hind tibial spur” of the chloropine genus 
Hippelates. 
We may conclude from these data that mutations do frequently 
bring about changes similar to those found in wild species. The 
general impression gained from considerable experience with both 
types of animals is that specific differences may be found in practically 
any character studied, and that the same is true of mutational differ- 
ences. Mutational differences may be of almost any degree, ranging 
from types that differ so slightly that statistical methods are necessary 
for the detection of any differences at all, to such extreme forms as 
the eyeless or wingless races. ‘‘Superficial’’ characters, such as eye- 
color or bristle-size, are affected as frequently as are such ‘‘funda- 
mental”? ones as the differentiation between the mesothoracic and 
metathoracic segments, the reaction to light, or the fertility or the 
viability of the individual. 
There are, however, marked differences between the diversity shown 
by the mutant races of a single species and the diversity shown by 
wild species of a small group. The species usually differ slightly from 
each other in innumerable characters, but are not strikingly different 
in any characters. The mutant races, on the other hand, are alike 
in most of the details of their structure, but often differ strikingly in a 
few characters. 
These facts do not indicate that specific and mutational differences 
are different in origin and kind. Any considerable experience with 
