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 Dichet, 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 Dichet 
gene. 
(9) A new allelomorph of Dichzt, called Extended, appeared in a 
plus selected line. It is argued that this mutation was not due to 
fractionation of the Dichet 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 Dichets, and of the not-Dichets pro- 
duced by long-continued mating together of Dichzts, 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; (b) 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. 
