212 



THE AMERICAN NATURALIST 



[Vol. L 



then, under certain special conditions of heterozygosis, 

 different regions of the same chromosome may differ in 

 regard to the frequency of crossing-over within them, for 

 different regions were not affected in the same way by 

 these factors ; it also shows that equal lengths of differ- 

 ent chromosomes may have different frequencies of cross- 

 ing-over, for these factors affected only group II appre- 

 ciably. 



In group III, crosses involving several combinations of 

 different factors have been made by Sturtevant, Bridges 

 and Dexter, but the order of none of the factors has until 

 recently been worked out by them nor has any consistent 

 general scheme been attempted. The information has, in 

 fact, not been adequate for this purpose, and much con- 

 fusion has also arisen on account of the great linkage 

 variation in this group, which seemed to occur very fre- 

 quently. Sturtevant, who, as stated in section I, discov- 

 ered the first case of linkage in the third group— namely, 

 that between pink and ebony— had determined the initial 

 positions of these factors, placing them at about 4 units 

 apart, and next Bridges, who had found kidney (eye), 

 had determined its position at about 15 units from pink, 

 though he did not determine the relation between kidney 

 and ebony. As a matter of fact, however, the kidney 

 determination had been made with flies in which there 

 was a greater frequency of crossing-over than in the ex- 

 periments of Sturtevant, and, as will appear later, in any 

 given experiment kidney is really nearer to pink than is 

 ebony. From time to time after this other mutants were 

 discovered (peach, Bridges, May, 1913; sepia, Wallace, 

 May, 1913; spineless, Bridges, May, 1913; deformed eye, 

 E. Cattell, 1913; band, Morgan, 1913; rough, Muller, 

 June, 1913 ; sooty, Sturtevant, Oct. 1913 ; spread, Dexter, 

 Nov. 1913; dichaete, Bridges, July, 1915), and the fact 

 that these mutants were members of the third chromo- 

 some group was determined (peach, sepia, spineless, band 

 and dichaete by Bridges ; deformed by Cattell ; rough by 

 Muller; and sooty by Sturtevant). The author mean- 



