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. rnelanogaster. 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 



