SPECIFIC DIFFERENCES VS. MUTATIONAL DIFFERENCES. 121 



In some of the Phoridse (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 (Chloropinae). 



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 Borhorus. 



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 Asteia and the agromyzine Phytomyza. 



In "short" D. immigrans 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 Borborinae. 



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 usaally 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 



