of the selection, and when it is recalled that during those five years over
125 new characters have appeared elsewhere it does not seem improbable that
factors also have appeared that modify the wings of this stock.

[Illustration: FIG. 84. Two flies showing beaded wings.]

(3) The third method is one that has been developed principally by East for
plants; also by MacDowell for rabbits and flies. The method does not claim
to prove that modifiers are present, but it shows why certain results are
in harmony with that expectation and can not be accounted for on the basis
that a factor has changed. Let me give an example. When a Belgian hare with
large body was crossed to a common rabbit with a small body the hybrid was
intermediate in size. When the hybrid was crossed back to the smaller type
it produced rabbits of various sizes in apparently a continuous series.
MacDowell made measurements of the range of variability in the first and in
the second generations.

_Classification in relation to parents based on skull lengths and ulna
lengths, to show the relative variability of two measurements and of
the first generation (F_1) and the back cross (B. C.)_

CHARACTER|GENERATION|-13|-12|-11|-10| -9| -8| -7| -6| -5| -4| -3| -2| -1|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | | | | | | |
skull { B.C. | | | | | | | | | | | | | 3|
Length of{ F_1 | | | | | | | | | | | | | |
ulna { B.C. | 1| | | | | 1| | 2| 3| 1| 2| 4| 4|

_same table continued_

CHARACTER|GENERATION| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| 11| 12|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | | | | | | | | 2| 2| 8| 5| 10| 7|
skull { B.C. | 6| 4| 13| 18| 42| 32| 38| 34| 16| 16| 8| 4| 3|
Length of{ F_1 | | 1| | | 2| | 1| 1| 1| 2| 2| 5| 3|
ulna { B.C. | 12| 11| 20| 26| 17| 19| 18| 15| 12| 13| 15| 11| 5|

_same table continued_

CHARACTER|GENERATION| 13| 14| 15| 16| 17| 18| 19| 20| 21| 22| 23| 24| 25|
---------+----------+---+---+---+---+---+---+---+---+---+---+---+---+---+
Length of{ F_1 | 3| 2| 2| | | | | | | | | | |
skull { B.C. | 1| | | | | | | | | | | | |
Le