selection would
then be due to a change in one or more of the factors themselves.
Johannsen's experiment is an example of this sort.

[Illustration: FIG. 83 a. Drosophila ampelophila with truncate wings.]

(2) The second method is one that is capable of _demonstrating_ that the
effects of selection are actually due to modifiers. It has been worked out
in our laboratory, chiefly by Muller, and used in a particular case to
demonstrate that selection produced its effect by isolating modifying
factors. For example, a mutant type called truncate appeared, characterized
by shorter wings, usually square at the end, (fig. 83a). The wings varied
from those of normal length to wings much shorter (fig. 83b). For three
years the mutant stock was bred from individuals having the shorter wings
until at last a stock was obtained in which some of the individuals had
wings much shorter than the body. By means of linkage experiments it was
shown that at least three factors were present that modified the wings.
These were isolated by means of their linkage relations, and their mutual
influence on the production of truncate wings was shown.

[Illustration: FIG. 83 b. Series of wings of different length shown by
truncate stock of D. ampelophila.]

An experiment of this kind can only be carried out in a case where the
groups of linked gens are known. At present Drosophila is the only animal
(or plant) sufficiently well known to make this test possible, but this
does not prove that the method is of no value. On the contrary it shows
that any claim that factors can themselves be changed can have no finality
until the claim can be tested out by means of the linkage test. For
instance, bar eye (fig. 31) arose as a mutation. All our stock has
descended from a single original mutant. But Zeleny has shown that
selection within our stock will make the bar eye narrower or broader
according to the direction of selection. It remains to be shown in this
case how selection has produced its effects, and this can be done by
utilizing the same process that was used in the case of truncate.

Another mutant stock called beaded (fig. 84), has been bred for five years
and selected for wings showing more beading. In extreme cases the wings
have been reduced to mere stumps (see stumpy, fig. 5), but the stock shows
great variability. It is probable here as Dexter has shown, that a number
of mutant factors that act as modifiers have been picked up in the course