376 CARNEGIE INSTITUTION OF WASHINGTON. 



It is important to note that whenever mutants are discarded, it is 

 because of their poor \'iability, or because of too great difficulty of 

 separation, etc., and not because they give results inconsistent with 

 those found with other more workable mutants. Our purpose in 

 keeping mutants is not to make a collection of them, but we keep 

 those that are useful in attacking special problems. 



Several of the stocks that were difficult to maintain, as they re- 

 quired selection in each generation, have been made self-perpetuating. 

 All first-chromosome stocks that have sterile females are now main- 

 tained by two new methods. One way is to cross the male of such 

 stock to "double-yellow" females (to be referred to later) ; another way 

 is to cross the male to a mutant that is at the same time a lethal and 

 prevents crossing-over throughout the first chromosome. The latter 

 mutant was discovered and its application pointed out by Professor 

 H. J. Muller. 



It is interesting to note that the stocks have apparently not been 

 reduced in vigor or in fertility, although some of the cultures have been 

 maintained for some 200 generations. The accompanying map gives 

 the position of about 125 loci, including corrections to November 7, 

 1921 . To make the maps more useful in everyday work, a new graphic 

 method is being planned for representing directly the percentage of 

 recombination that corresponds to each particular section or region 

 of the chromosomes. 



The more accurate locations are given to the nearest half unit; 

 the less accurate locations to the nearest unit. The more useful 

 mutants are indicated by *; the mutants with medium usefulness 

 are marked by -f; the mutants that are unmarked are useful only 

 in special experiments. Dominants are capitalized. 



The value of these maps for the purpose for which they were con- 

 structed is contmually becoming manifest. There is little excuse in 

 misunderstanding what they stand for, but since they have been 

 misunderstood by one or two writers, the following statement may 

 not seem superfluous. The cross-over values on which the distances 

 on the maps are based are known to be influenced by various condi- 

 tions, such as temperature, age, and genetic factors. Nevertheless, 

 in practice cross-over values are usually found to agree closely with 

 the distances given on these maps. This reassures us that up to the 

 present time no seriously complicatmg factors have been overlooked. 

 When complicating factors are known to be present, and have been 

 studied, it has always been found that the sequence of the loci is not 

 changed. The effect of these factors is on the frequency of crossing- 

 over and does not produce any change in the order of the genes. 

 Therefore, while the distances are subject to some change, as we have 

 more than once pointed out, the order of the genes remains unchanged. 



As has already been reported, the only other species of Drosophila 

 that ha,s been found to cross with melanogaster is D. simulans. The 



