52 AN ANALYSIS OF THE EFFECT OF SELECTION. 



(7) In one case at least one third-chromosome modifier was shown 

 to exist and to cross over from Dichset, which must lie to the left of it. 



(8) Two third-chromosome lethals were obtained. These were 

 shown to be new mutations, not due to fractionation of the Dichset 

 gene. 



(9) A new allelomorph of Dichset, called Extended, appeared in a 

 plus selected line. It is argued that this mutation was not due to 

 fractionation of the Dichset gene, and was not influenced by the selec- 

 tion that was carried on. 



(10) Another character, somatically indistinguishable from Ex- 

 tended, was shown to be due to a recessive second-chromosome gene. 



(11) A study of unselected Dichsets, and of the not-Dichsets pro- 

 duced by long-continued mating together of Dichsets, is shown to fur- 

 nish evidence against the view that allelomorphs are contaminated in 

 heterozygotes. 



(12) A general discussion of the selection problem is divided into 

 three parts: (a) an attempt is made to clear up certain current mis- 

 understandings and disagreements as to what questions are really at 

 issue; (6) cases cited as evidence for contamination of allelomorphs 

 are discussed in detail, and the conclusion is drawn that contamina- 

 tion is unproved and is an unnecessary hypothesis, with some direct 

 evidence against it; (c) certain specific objections are raised to argu- 

 ments made by Castle on the basis of his experiments with hooded 

 rats. 



